Revert 166464 - Turn overdraw metrics on only when about:tracing is recording.

Revert 166464 - Turn overdraw metrics on only when about:tracing is recording.

Now that we're in chromium, we can tell if about:tracing is enabled ot not!
Turn on overdraw metrics, and their expensive computations, only when
about:tracing is turned on.

When we do turn them on, we don't want the performance characteristics of
the system to suddenly change, or the tracing is not very meaningful! So, we
track the number of pixels read, instead of written, for overdraw (which
should mostly be the same with the new rasterScale feature). This
computation is very cheap compared to the old one.

R=jamesr
BUG=119126
Review URL: https://codereview.chromium.org/11293143

TBR=danakj@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=166477

[scottmg, 2012-11-07 18:40:29]

http://build.chromium.org/p/chromium.memory/builders/Chromium%20OS%20ASAN%20T...