Run Pepper 2D flush ack code on ViewInitiatedPaint.
The Pepper 2D flush ack was originally executed on ViewFlushedPaint (UpdateRect ack) both to throttle it and so that it was certain that the TransportDIB that was shared to the browser was not in use anymore. It makes sense to move the flush ack to ViewInitiatedPaint (compositor commit completed) because that's consistent with pepper 3d, handles throttling at least as well, and it's not sharing TransportDIBs with the browser anymore so that doesn't matter.
BUG=438646
Review URL: https://codereview.chromium.org/
784443002
Cr-Commit-Position: refs/heads/master@{#306953}