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I plan to use the CPU Usage functions in other non-Youtube related perf
tests, so moving these helper functions from the Youtube perf class to the
base perf class.  Also some minor style edits to the Youtube perf test
code.

BUG=None
TEST=None

Review URL: http://codereview.chromium.org/8585050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110633 0039d316-1c4b-4281-b951-d872f2087c98