Instrumenting AccountTracker::StartFetchingUserInfo to locate the source of jankiness

Instrumenting AccountTracker::StartFetchingUserInfo to locate the source of jankiness.

Method AccountTracker::StartFetchingUserInfo is executed in browser's UI thread, and it causes jank.
In Canary 40.0.2201.2 on Win, 19.9 jph is caused by AccountTracker::StartFetchingUserInfo,
which average execution time in UI thread is 227 ms. On Mac, we have 17.7 jph and 71.1ms avg. I need to instrument
the code inside it to find out which part causes jank.

This is a mechanical change that adds instrumentation required to locate the
source of jankiness (i.e. a long-running fragment of code executed as a part of
the task that causes jank) in the code. See the bug for details on what kind of
jank we are after.
A number of similar CLs were landed, and none of them caused issues. They've
helped to find and fix janky code. The code of the instrumentation is highly
optimized and is not expected to affect performance. The code simply creates a
diagnostic task which is identical to ones created by PostTask or IPC message
handlers. The task gets created only in developer build and in Canary channel.

BUG=422460
Committed: https://crrev.com/e924411b95fa49f84793ff8d33fc107f9a7a0a3b
Cr-Commit-Position: refs/heads/master@{#302140}

[vadimt, 2014-10-30 18:18:22]

vadimt@chromium.org changed reviewers:
+ rogerta@chromium.org

[vadimt, 2014-10-30 18:24:41]

rogerta@, please provide OWNER's approval

[zel, 2014-10-30 18:45:17]

zelidrag@chromium.org changed reviewers:
+ zelidrag@chromium.org

[zel, 2014-10-30 18:45:17]

lgtm

[vadimt, 2014-10-30 18:57:25]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-10-30 18:59:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/690073002/1

[commit-bot: I haz the power, 2014-10-30 19:57:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-30 19:58:12]

Patchset 1 (id:??) landed as
https://crrev.com/e924411b95fa49f84793ff8d33fc107f9a7a0a3b
Cr-Commit-Position: refs/heads/master@{#302140}