Adding instrumentation to locate the source of jankiness.

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.

BUG=456327, 456331, 456373
TEST=none
TBR=mmenke, erikwright

Committed: https://crrev.com/b60049a86e45b0160f378624c5dfd99d667c5262
Cr-Commit-Position: refs/heads/master@{#315173}

[Peter Kasting, 2015-02-07 00:43:49]

pkasting@chromium.org changed reviewers:
+ erikwright@chromium.org, mmenke@chromium.org

[Peter Kasting, 2015-02-07 00:44:31]

TBRing for mechanical changes:

erikwright: content/browser/net/ OWNERS
mmenke: rest

[Peter Kasting, 2015-02-07 01:15:46]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2015-02-07 01:16:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/894903004/1

[commit-bot: I haz the power, 2015-02-07 03:25:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-07 03:26:17]

Patchset 1 (id:??) landed as
https://crrev.com/b60049a86e45b0160f378624c5dfd99d667c5262
Cr-Commit-Position: refs/heads/master@{#315173}