This replaces the use of painting in RenderImage to display an image's alt text
with an implementation in shadow DOM. This initial implementation is close in
appearance to the legacy display of alt-text but will ultimately move closer to
the one seen in Firefox and described in http://hixie.ch/specs/alttext.

The alt-text and broken-image icon is now rendered as:

    <style>
        #alttext-container {
        overflow: hidden;
        border: 1px solid silver;
        padding: 1px;          
        display: inline-block;        
        }                                 
        #alttext { display: none; overflow: hidden;}
    </style>                                    
    <div id="alttext-container">                  
        <img src="data:png, [broken-image-icon]" width="16" height="16" align="left" style="margin: 0px">
        <div id="alt-text">Alt text in here</div>         
    </div>                                                  

Some notes on the way the fallback content is now rendered: 

    - The fallback content is rendered inside an inline-block so it does not calculate 
      its dimensions the way a replaced element (i.e. an image) would (as defined by
      http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width). So where one of width 
      or height is auto but the other is not, the fallback content will not exactly match 
      the dimensions calculated by RenderImage in error mode currently. We do a modest
      imitation of the logic in quirks mode, but in strict mode it will behave exactly like an inline-block.

    - Where the image has no src attribute and no alt attribute RenderImage.cpp
      still looks after the painting of the element - no fallback content is generated
      and no broken image is displayed. This is consistent with existing behaviour.

    - The only image resource requests that still use the synchronous load path are
      those where the item is cached (and didn't error out), where it's a data: source,
      where its the fallback image for an object element, or where it's the image 
      for a main resource load. All other image loads are now asynchronous so that
      fallback content can be loaded outside the style recalc phase.


    - Instead of aborting an image resource request when the src element is empty
      (i.e. src='') we now allow the request to go through so that it can fail and
      invoke the fallback content in HTMLImageLoader.cpp.   

    - As you can see in the new result for fast/borders/rtl-border-05.html, since
      the alt-text is displayed as an inline-block it no longer artificially shrinks
      any border on the element to the broken-image icon.   

Some notes on the rebaselined test results:                 

    - I've modified inspector/network/network-image-404.html to output the state of
      both the resource requests, i.e. the failed one and the resource request for the
      data:png to display the broken image icon.            

    - I've added a missing support file for fast/css/counters/complex-before.html -
      its absence meant that the result was polluted by the behaviour of broken image
      rendering.                                            

    - Likewise for fast/images/imagemap-polygon* tests - our new rendering of failed
      image loads was interfering with an assumption in the tests that a broken image
      still painted a RenderImage. So I've removed the src attribute to allow the
      assumption hold (img elements without a src attribute are painted by
      RenderImage).

    - http/tests/security/local-image-from-remote-whitelisted.html has been masking
      a bug - blink does not load the image even though it is whitelisted. I am
      rebaselining the test to reflect the failure revealed by this CL and tracking 
      a fix under crbug.com/410949.                         

    - I have altered fast/forms/state-restore-to-non-edited-controls.html to wait 
      100ms before submitting the form as the image load in the input element is now 
      asynchronous.

Review URL: https://codereview.chromium.org/481753002

git-svn-id: svn://svn.chromium.org/blink/trunk@184770 bbb929c8-8fbe-4397-9dbb-9b2b20218538