Measure IPC latency for audio...

Measure IPC latency for audio
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.


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=15063

[scherkus (not reviewing), 2009-04-30 02:15:10]

LGTM

just curious why we moved away from high res timers... is it because IPC message
logging uses low res timers?

http://codereview.chromium.org/99213/diff/1/4
File chrome/browser/renderer_host/audio_renderer_host.cc (right):

http://codereview.chromium.org/99213/diff/1/4#newcode167
Line 167: #ifdef IPC_MESSAGE_LOG_ENABLED
do you know how we get this defined?

http://codereview.chromium.org/99213/diff/1/4#newcode168
Line 168: base::Time tick_start = base::Time::Now();
curious... why not HighResNow?