cc: Turn overdraw metrics on only when about:tracing is recording.

cc: 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. 

TBR=jamesr 
BUG=119126
Relanding: https://codereview.chromium.org/11293143/

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=166521