Pyauto perf tests do not wait for idle CPU on Mac.
Pyauto perf tests do not wait for idle CPU on Mac.

Waiting for an idle CPU in the pyauto perf tests requires a call to
/proc/stat, which doesn't exist on Mac platforms.  This change disables
the wait for an idle CPU on Mac systems, and adds a TODO indicating that
it needs to be implemented properly for Mac/Windows.

BUG=None
TEST=Verified the pyauto perf tests run on a local Macbook Air with this change.

Review URL: https://chromiumcodereview.appspot.com/9442004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@123297 0039d316-1c4b-4281-b951-d872f2087c98

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