Add user action metric for UI DevTools http server connections

Add user action metric for UI DevTools http server connections

Introduce the user action metric UI_DevTools_Connect, which
is recorded whenever a connection to a UI DevTools
http server is established.

BUG=746562

[tdanderson, 2017-07-20 15:37:05]

The CQ bit was checked by tdanderson@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-07-20 15:37:14]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[tdanderson, 2017-07-20 15:41:22]

tdanderson@chromium.org changed reviewers:
+ mpearson@chromium.org, sadrul@chromium.org

[tdanderson, 2017-07-20 15:41:23]

Sadrul and Mark, can you please take a look?

[commit-bot: I haz the power, 2017-07-20 17:19:40]

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

[commit-bot: I haz the power, 2017-07-20 17:19:41]

Dry run: Try jobs failed on following builders:
  linux_android_rel_ng on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/linux_androi...)

[Mark P, 2017-07-20 21:37:49]

lgtm