Notify listeners on bitmaprenderer context changes

This CL addresses the bug in HTMLCanvasElement::getSourceImageForCanvas() caused
by the uninitialized value of |status|. With the corrected status, we can send
captured frame to canvas listeners.

BUG=679610
TEST=Added layout test checking for a single frame capture and content_browsertest
for a continuous capture.

Review-Url: https://codereview.chromium.org/2683343002
Cr-Commit-Position: refs/heads/master@{#452743}
5 files changed