Improve download performance statistics.

Improve download performance statistics.

I've removed the questionable Download.BandwidthNetwork measurement, and
replaced it with measurements designed to answer the question "What is the
bottleneck for a download?"  Specfically, this includes two new statistics: 

* Download.DiskBandwidthUsedPercentage: The percentage of the
  available disk bandwith that was actually used.  100% implies that
  the disk was the performance bottleneck.
* Download.ResourceHandlerBlockedPercentage: The percentage of the
  time it was alive that the DownloadResourceHandler was throttled for
  performance reasons.  0% implies that the network was the
  performance bottleneck.

R=benjhayden@chromium.org


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=145320

[Randy Smith (Not in Mondays), 2012-07-02 16:53:36]

Ben: I had a chat with Jim Roskind as part of the histograms.xml code review,
and decided to tweak the stats some as per the issue description.  PTAL?

[benjhayden, 2012-07-02 17:19:47]

lgtm

[commit-bot: I haz the power, 2012-07-02 17:39:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdsmith@chromium.org/10695062/2001

[commit-bot: I haz the power, 2012-07-02 20:22:31]

Try job failure for 10695062-2001 (retry) on win_rel for steps "browser_tests,
content_unittests".
It's a second try, previously, steps "browser_tests, content_unittests" failed.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2012-07-03 15:26:16]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdsmith@chromium.org/10695062/5002

[commit-bot: I haz the power, 2012-07-03 16:53:33]

Change committed as 145320