Add support for tracing in browser tests.

Add support for tracing in browser tests.

This CL adds support for collecting tracing data during the execution of browser tests. It is currently not possible to do so, yet it is very useful to be able to collect this data and analyze it without having to step through with debugger.

BUG=

Committed: https://crrev.com/047af26ba1bb8dd0cb74c132b2578eb91d8c7076
Cr-Commit-Position: refs/heads/master@{#292156}

[nasko, 2014-08-26 22:16:53]

nasko@chromium.org changed reviewers:
+ jam@chromium.org

[nasko, 2014-08-26 22:16:53]

Hey John,
Can you review this CL for me?
Thanks,
Nasko

[jam, 2014-08-27 03:33:59]

lgtm, nice

https://codereview.chromium.org/506113002/diff/20001/content/public/common/co...
File content/public/common/content_switches.cc (right):

https://codereview.chromium.org/506113002/diff/20001/content/public/common/co...
content/public/common/content_switches.cc:452: const char kEnableTracing[] =
"enable-tracing";
nit: here and below, almost all flags have the '=' at the same column, please
follow that

[nasko, 2014-08-27 13:53:39]

The CQ bit was checked by nasko@chromium.org

[commit-bot: I haz the power, 2014-08-27 13:54:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/nasko@chromium.org/506113002/40001

[commit-bot: I haz the power, 2014-08-27 14:55:00]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_chromium_rel_swarming on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[commit-bot: I haz the power, 2014-08-27 15:34:56]

Committed patchset #3 (id:40001) as b61c74ca95d8c5e9d287baf3f2c5b60508081a62

[commit-bot: I haz the power, 2014-09-10 02:51:49]

Patchset 3 (id:??) landed as
https://crrev.com/047af26ba1bb8dd0cb74c132b2578eb91d8c7076
Cr-Commit-Position: refs/heads/master@{#292156}