Clean up RenderWidgetHostImpl::WaitForSurface
Remove comments talking about backing stores, because they are no
longer a thing.
Don't break if WaitForSingleTaskToRun returns false, because that function
only promises to wait "at most" the timeout specified -- it could return
immediately, with no task having ran.
Change the return on success to a break, since the return will miss the
UMA statistics.
Use Now() for the UMA statistics, because the path where a surface is
found will not update the time to reflect the time it took to wait for the
message, and to put the frame on screen.
BUG=
Review URL: https://codereview.chromium.org/
844493002
Cr-Commit-Position: refs/heads/master@{#310442}