Calling writePixels on a canvas using DisplayList2DCanvas was ignoring the call (canvas.pixels == null)
To make this work, we need to fallback to a raster canvas to be able to directly access the canvas' pixels.

To be more consistent, the method willReadback has be renamed to willAccessPixels.

Fixes the following layout-tests with --enable-display-list-2d-canvas:
  fast/canvas/canvas-imageSmoothingEnabled.html [ Failure ]
  fast/canvas/canvas-putImageData.html [ Failure ]
  fast/canvas/webgl/tex-image-and-sub-image-2d-with-image-rgb565.html [ Failure ]
  fast/canvas/webgl/tex-image-and-sub-image-2d-with-image-rgba4444.html [ Failure ]
  fast/canvas/webgl/tex-image-and-sub-image-2d-with-image-rgba5551.html [ Failure ]
  fast/canvas/webgl/tex-image-and-sub-image-2d-with-image.html [ Failure ]
  fast/canvas/canvas-imageSmoothingEnabled-patterns.html [ ImageOnlyFailure ]

BUG=401624

Review URL: https://codereview.chromium.org/455863002

git-svn-id: svn://svn.chromium.org/blink/trunk@179969 bbb929c8-8fbe-4397-9dbb-9b2b20218538