Prevent <canvas> destructor from causing nested garbage collection

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

[Justin Novosad, 2014-05-28 17:13:19]

PTAL

[Stephen White, 2014-05-28 17:26:10]

LGTM, bots willing.

[Justin Novosad, 2014-05-28 17:49:31]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2014-05-28 17:49:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/junov@chromium.org/303803002/1

[commit-bot: I haz the power, 2014-05-28 18:27:25]

Change committed as 174984

[Julien - ping for review, 2014-05-30 21:41:53]

A revert of this CL has been created in
https://codereview.chromium.org/307013006/ by jchaffraix@chromium.org.

The reason for reverting is: 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%20Te....