[GCM] Add port 443 fallback logic and histograms

[GCM] Add port 443 fallback logic and histograms

Alternate between the default (5228) endpoint and the fallback (443) endpoint.
Also introduce histograms for tracking endpoint and proxy success rate.

Relanding due to compile issue.
Original review: https://codereview.chromium.org/205343003/

BUG=351890
TBR=jianli@chromium.org, fgorski@chromium.org, asvitkine@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=258997

[Nicolas Zea, 2014-03-24 17:57:13]

The CQ bit was checked by zea@chromium.org

[commit-bot: I haz the power, 2014-03-24 17:58:22]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/zea@chromium.org/206873006/1

[commit-bot: I haz the power, 2014-03-24 19:17:13]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-24 19:17:13]

Try jobs failed on following builders:
  tryserver.chromium on linux_chromium_rel

[Nicolas Zea, 2014-03-24 19:48:30]

Committed patchset #1 manually as r258997 (presubmit successful).

[fgorski, 2014-03-24 23:17:32]

lgtm