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With this patch there's totally 5 measurements for audio IPC
1. Total round trip time
   browser request a packet until it gets a packet
2. Browser receive latency
   time for a packet to arrive browser from renderer
3. Browser process time
   time for browser to process the packet
4. Renderer receive latency
   time for a packet to arrive renderer from browser
5. Renderer process time
   process time on a packet in renderer

They are all using low resolution timer with accuracy of ~1-15ms.
Since we have a budget of ~200ms (for now) without clicks, I think
the accuracy shouldn't be a big problem, unless for case 1 where
we want to measure exact amount of clicks.

Review URL: http://codereview.chromium.org/99213

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@15063 0039d316-1c4b-4281-b951-d872f2087c98