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

This CL instruments code invoked from URLRequestFileJob::DidOpen, which is responsible for 15.3 jph.

BUG=423948
TBR=mmenke@chromium.org

Committed: https://crrev.com/8ada724c275215eb98e911624edb5abca763fd36
Cr-Commit-Position: refs/heads/master@{#301480}

[vadimt, 2014-10-24 23:27:51]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-10-24 23:29:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/675303004/1

[commit-bot: I haz the power, 2014-10-25 00:29:40]

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

[commit-bot: I haz the power, 2014-10-25 00:29:42]

Try jobs failed on following builders:
  android_dbg_tests_recipe on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/android_dbg_tes...)

[vadimt, 2014-10-27 22:23:30]

The CQ bit was checked by vadimt@chromium.org

[commit-bot: I haz the power, 2014-10-27 22:27:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/675303004/1

[commit-bot: I haz the power, 2014-10-27 23:24:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-27 23:24:38]

Patchset 1 (id:??) landed as
https://crrev.com/8ada724c275215eb98e911624edb5abca763fd36
Cr-Commit-Position: refs/heads/master@{#301480}