Adding instrumentation to locate the source of jankiness.

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. 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.
Landing as TBR since this is a mechanical, safe and temporary change.

BUG=422460
TBR=isherman@chromium.org, caitkp@chromium.org

Committed: https://crrev.com/1850ee0bdf4e7654803cf846b5326c8e33dc0810
Cr-Commit-Position: refs/heads/master@{#299198}

[vadimt, 2014-10-10 20:06:37]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-10-10 20:07:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647123002/1

[commit-bot: I haz the power, 2014-10-10 21:52:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-10 21:53:27]

Patchset 1 (id:??) landed as
https://crrev.com/1850ee0bdf4e7654803cf846b5326c8e33dc0810
Cr-Commit-Position: refs/heads/master@{#299198}