DevTools: [CanvasProfiler] Measure replay time in backend instead of front-end.

We used to show replay time measured in front-end, which included time spent on passing the data from backend to front-end.
Now the time is measured in backend, and gives a first bit of information about performance.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/19276004

git-svn-id: svn://svn.chromium.org/blink/trunk@154568 bbb929c8-8fbe-4397-9dbb-9b2b20218538