[UMA] Fix a crash on logging user actions posted from a non-UI thread.

[UMA] Fix a crash on logging user actions posted from a non-UI thread.

Along the way, add DCHECKs to verify that user actions are always posted from
the UI thread.

BUG=360271
TEST=none
R=asvitkine@chromium.org, avi@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=262885

[Ilya Sherman, 2014-04-09 00:41:41]

Alexei, PTAL at //base/metrics.
Avi, PTAL at //content.

Thanks!

[Avi (use Gerrit), 2014-04-09 01:13:23]

lgtm

[Alexei Svitkine (slow), 2014-04-09 15:03:43]

LGTM with nits

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc
File base/metrics/user_metrics.cc (right):

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:37: void Record(const std::string& action) {
Nit: Match order of the corresponding functions below.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:43: 
Nit: Remove extra line.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:47: base::ThreadChecker thread_checker_;
Nit: Put this above callbacks to correspond to the order it's used in the
functions.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:48: };
Nit: DISALLOW_COPY_AND_ASSIGN()?

[Ilya Sherman, 2014-04-09 22:42:39]

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc
File base/metrics/user_metrics.cc (right):

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:37: void Record(const std::string& action) {
On 2014/04/09 15:03:44, Alexei Svitkine wrote:
> Nit: Match order of the corresponding functions below.

Done.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:43: 
On 2014/04/09 15:03:44, Alexei Svitkine wrote:
> Nit: Remove extra line.

Done.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:47: base::ThreadChecker thread_checker_;
On 2014/04/09 15:03:44, Alexei Svitkine wrote:
> Nit: Put this above callbacks to correspond to the order it's used in the
> functions.

Done.

https://codereview.chromium.org/229873002/diff/1/base/metrics/user_metrics.cc...
base/metrics/user_metrics.cc:48: };
On 2014/04/09 15:03:44, Alexei Svitkine wrote:
> Nit: DISALLOW_COPY_AND_ASSIGN()?

Done.

[Ilya Sherman, 2014-04-09 22:42:53]

The CQ bit was checked by isherman@chromium.org

[commit-bot: I haz the power, 2014-04-09 22:43:37]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/isherman@chromium.org/229873002/20001

[commit-bot: I haz the power, 2014-04-10 01:12:37]

Change committed as 262885