Add error handling of beginRecording

Add error handling of beginRecording

- End recording when the chrome://tracing window is closed or reloaded,
  otherwise we can't start tracing again.
- Handle '##ERROR##' from /json/categories and /json/begin_recording
  requests (required a C++ side change). This will popup an error box
  if tracing is started from the second chrome://tracing window while
  the first chrome://tracing window is tracing.

BUG=chromium:331286
R=nduca@chromium.org

Committed: https://code.google.com/p/trace-viewer/source/detail?r=1084

[Xianzhu, 2014-01-03 23:05:05]

https://codereview.appspot.com/47660043/diff/1/src/about_tracing/begin_record...
File src/about_tracing/begin_recording.js (left):

https://codereview.appspot.com/47660043/diff/1/src/about_tracing/begin_record...
src/about_tracing/begin_recording.js:24: resolver.reject(new Error('At ' + path
+ ' error ' + req.status));
Removed req.status because the status is always 200.

[nduca, 2014-01-03 23:16:29]

lgtm

[Xianzhu, 2014-01-03 23:23:30]

Committed patchset #1 manually as r1084 (presubmit successful).

[Xianzhu, 2014-01-04 00:08:09]

The chromium side change also needs an approval:
https://codereview.chromium.org/117323008/.