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This is because collecting the stats often crashes.

Added a guard to prevent the collection of diagnostics on multiple threads simultaneously.

Renamed GPUInfo::Progress to GPUInfo::Level.

TEST=try, about:gpu does not cause concurrent diagnostics collection
BUG=none

Review URL: http://codereview.chromium.org/6364013

TBR=apatrick@chromium.org
Review URL: http://codereview.chromium.org/6370013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72707 0039d316-1c4b-4281-b951-d872f2087c98