2D Canvas: Refactor code re-attempting to allocate an imageBuffer.

m_hasCreatedImageBuffer prevents HTMLCanvasElement::buffer()
from continuously re-attempting to allocate an imageBuffer after the first
attempt failed. This has huge implications for performance in low memory
conditions.

m_hasCreatedImageBuffer was misnamed, so it ended-up being misused in several
places.

This CL renames m_hasCreatedImageBuffer to m_didFailToCreateImageBuffer, and
then m_didFailToCreateImageBuffer is used in only relevant places.

TEST=fast/canvas/canvas-extremely-large-dimensions.html

Review URL: https://codereview.chromium.org/44253005

git-svn-id: svn://svn.chromium.org/blink/trunk@162330 bbb929c8-8fbe-4397-9dbb-9b2b20218538