DevTools: fix flakeyness of inspector/tracing/decode-resize.html

DevTools: fix flakeyness of inspector/tracing/decode-resize.html

This keeps timeline recording for a few more frames once we've
done all image update. This is still somewhat fragile, but there seems
to be not much we can do without exposing additional interfaces
from the CC, given that image decode happens on a separate thread
with no completion signals sent to main thread.

BUG=498539
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196874

[caseq, 2015-06-10 13:49:56]

caseq@chromium.org changed reviewers:
+ yurys@chromium.org

[yurys, 2015-06-10 13:54:01]

lgtm

[caseq, 2015-06-10 13:54:22]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2015-06-10 13:54:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174983002/1

[commit-bot: I haz the power, 2015-06-10 14:58:28]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196874