DevTools: [CanvasProfiler] Improve replay time measuring.

Right now replay time value includes time spent on auxiliary things, like
replaying resources from previous frames. With this patch we do a "warm-up"
first, before measuring the actual replay time.

With this patch the replay time value is far more accurate and precise.

BUG=
R=caseq@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22870012

git-svn-id: svn://svn.chromium.org/blink/trunk@163327 bbb929c8-8fbe-4397-9dbb-9b2b20218538