GitLab

Previously, we didn't properly clear all the network caches.
1. DNS and predictor caches were left in tact. This was fixed by calling the
   net benchmarking extension methods.
2. Live resources in the memory cache were left in tact. This is fixed by
   navigating to about:blank to destroy the old renderer.

Also, --enable-net-benchmarking is supported on the stable channel, so we can
remove the workaround.

With this, cold loads are consistent on Android:

*RESULT cold_times: page_load_time= [4637,4493,4479,4564,4611,4549] ms
Avg cold_times: 4555.500000ms
Sd  cold_times: 62.602716ms
RESULT warm_times_by_url: http___www.youtube.com_watch?v_LIJOhk6FscY= [2122,2104,2079,2094] ms
Avg warm_times_by_url: 2099.750000ms
Sd  warm_times_by_url: 18.043928ms

BUG=

Review URL: https://codereview.chromium.org/165193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251252 0039d316-1c4b-4281-b951-d872f2087c98