Revert 159400 "REGRESSION: Text INPUT inside a LABEL associated ..."

We believe this change caused crbug.com/306882. This is an optimistic revert to help fix the Dev branch.

> REGRESSION: Text INPUT inside a LABEL associated to a radio button can't get focus by mouse press.
> 
> This is a regression by Blink r153039 [1], which made radio buttons
> mouse-focusable.
> 
> We fix this regression by improving specification conformance.
> The specification [2] says:
> > The activation behavior of a label element for events targeted at interactive
> > content descendants of a label element, and any descendants of those
> > interactive content descendants, must be to do nothing.
> 
> We didn't have such behavior. Implementing it fixes the regression beucase mouse
> press on the text <input> does nothing for <label>.
> 
> The new behavior is compatible with IE.
> 
> Implementation:
> We introduce HTMLElement::isInteractiveContent virtual function. Interactive
> content elements defined by the specification should override it so that it
> returns true. HTMLLabelElement checks if a click event is inside an interactive
> content.
> 
> [1] http://src.chromium.org/viewvc/blink?revision=153039&view=revision
> [2] http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#the-label-element
> 
> BUG=305751
> TEST=automated
> 
> Review URL: https://codereview.chromium.org/26689007

TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/27222004

git-svn-id: svn://svn.chromium.org/blink/trunk@159596 bbb929c8-8fbe-4397-9dbb-9b2b20218538