Call PNGDecoder setHasAlpha only when needed

Recover the prior code that first checked for trivial alpha before
ever calling ImageFrame::setHasAlpha().

BUG=

Review URL: https://chromiumcodereview.appspot.com/17801002

git-svn-id: svn://svn.chromium.org/blink/trunk@153052 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert 152954 "Add web workers idle notifications for scripting ..."

Need more work.

> Add web workers idle notifications for scripting engine.
> 
> The patch adds message queue monitoring and periodically makes calls to
> ScriptController idle notification mechanism.
> It is done when the message queue is empty and there's plenty of time before
> the next scheduled event.
> 
> No observable JS behavior change besides a smaller memory footprint, so no test.
> 
> TBR=yurys@chromium.org
> BUG=241815
> 
> Review URL: https://codereview.chromium.org/17585004

TBR=dimich@chromium.org,yurys@chromium.org

Review URL: https://codereview.chromium.org/17815003

git-svn-id: svn://svn.chromium.org/blink/trunk@153053 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce toAttr functions, and use them instead of static_cast<>.

BUG=
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/17806002

git-svn-id: svn://svn.chromium.org/blink/trunk@153054 bbb929c8-8fbe-4397-9dbb-9b2b20218538