Revert of Prevent <canvas> destructor from causing nested garbage collection

Revert of Prevent <canvas> destructor from causing nested garbage collection (https://codereview.chromium.org/303803002/)

Reason for revert:
This change is the cause of a
leak in PhishingDOMFeatureExtractorTest.ScriptAndImageFeatures
under LSAN. See
http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%281%29/builds/2642/steps/browser_tests/logs/ScriptAndImageFeatures

Original issue's description:
> Prevent <canvas> destructor from causing nested garbage collection
> 
> In the destruction of HTMLCanvasElement, we call
> HTMLCanvasElement::existingDrawingContext(), whose behavior was changed
> incorrectly in r162330 and was triggering the allocation of an ImageBuffer
> during tear-down.  This is not only wasteful, but it can trigger a
> garbage collection because the creation of an ImageBuffer will increase
> the amount of externallay allocated memory that is declared to V8.
> Triggering a GC during the finalization of a garbage collected object
> is bad.
> 
> BUG=378289
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174984

TBR=senorblanco@chromium.org,junov@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=378289
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175157

[Julien - ping for review, 2014-05-30 21:41:54]

Created Revert of Prevent <canvas> destructor from causing nested garbage
collection

[Julien - ping for review, 2014-05-30 21:59:11]

Committed patchset #1 manually as r175157 (tree was closed).

[Julien - ping for review, 2014-05-30 22:16:11]

A revert of this CL has been created in
https://codereview.chromium.org/301243009/ by jchaffraix@chromium.org.

The reason for reverting is: This isn't the right CL, was off-by-one..