Streamline the presentation of ImageBitmapRenderingContext

This change adds an ImageLayerBridge class that provides a direct
compositing path for displaying ImageBitmap objects, without
going through a blink paint pass.  As a consequence, there are
no more GPU readbacks when the source of the image is
GPU-accelerated.

In canvas rendering context classes, the notion of being
accelerated had to be decoupled from the notion of being
composited.  This is because non-accelerated canvases are
are directly composited with ImageBitmapRenderingContext.

BUG=696224, 585607
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2738573002
Cr-Commit-Position: refs/heads/master@{#462634}
27 files changed