Update to reflect WhatWG changes

[ccameron-chromium]

In WhatWG review, a handful of changes were made to this proposal. Update to reflect these changes, to avoid confusion.

• Note that WebGL/GPU parts are in separate review from Canvas2D changes
• Update ImageData interface to use an attribute
• Update getImageData and createImageData methods to use canvas's color space by default
• Move ImageBitmap changes to be discussed under WebGL (similarly not part of the Canvas2D changes)