Check if precaching is allowed before reporting related metrics

Check if precaching is allowed for the current browser context before
measuring any precache-related metrics.

BUG=309216
Committed: https://crrev.com/387f633d1205ae65bacb5e4a0c4d174f32862874
Cr-Commit-Position: refs/heads/master@{#292677}

[bengr, 2014-08-28 22:38:35]

bengr@chromium.org changed reviewers:
+ bengr@chromium.org, mef@chromium.org

[bengr, 2014-08-28 22:38:35]

mef: chrome/browser/net/*
bengr: everything else.

[bengr, 2014-08-28 22:39:27]

lgtm

[mef, 2014-08-29 12:21:38]

lgtm, but please update issue subject to be complete sentence.
Also may need to remove BUG= from its text.

Does it make sense to add some tests?

[bengr, 2014-08-29 16:51:03]

The CQ bit was checked by bengr@chromium.org

[commit-bot: I haz the power, 2014-08-29 16:52:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tanviraumi@google.com/514353002/1

[bengr, 2014-08-29 16:58:16]

The CQ bit was unchecked by bengr@chromium.org

[bengr, 2014-08-29 17:06:19]

bengr@chromium.org changed reviewers:
+ battre@chromium.org, bengr@google.com

[bengr, 2014-08-29 17:06:19]

battre: DEPS

[battre, 2014-08-29 17:15:56]

lgtm

[bengr, 2014-08-29 17:16:43]

The CQ bit was checked by bengr@chromium.org

[commit-bot: I haz the power, 2014-08-29 17:17:48]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tanviraumi@google.com/514353002/1

[commit-bot: I haz the power, 2014-08-29 18:11:30]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_x64_rel_swarming on tryserver.chromium.win
(http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_x64_...)

[commit-bot: I haz the power, 2014-08-29 19:04:53]

Committed patchset #1 (id:1) as 0bba4f4bd486905fdbc5a29c9eec03502680e912

[commit-bot: I haz the power, 2014-09-10 03:10:21]

Patchset 1 (id:??) landed as
https://crrev.com/387f633d1205ae65bacb5e4a0c4d174f32862874
Cr-Commit-Position: refs/heads/master@{#292677}