Implement sampling profiler (chromium side change)

Trace-viewer side change is here: https://codereview.appspot.com/39310043

Screenshot of the profiler: http://haraken.info/null/sampling_tracing.png

Design document: https://docs.google.com/a/google.com/document/d/1j39sbA9ECTwFFlxbhSD1EvJnGfYeS5EUJ05yxuku6VY/edit

TBR=phajdan.jr
NOTRY=true

Review URL: https://codereview.chromium.org/109933006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242671 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242671