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

Review URL: https://codereview.chromium.org/307013006

git-svn-id: svn://svn.chromium.org/blink/trunk@175157 bbb929c8-8fbe-4397-9dbb-9b2b20218538