Allow TracingController output trace data to specified file

Sometimes we need to let TracingController output traces to specified
file, e.g. trace-startup (which will be migrated from TraceController
to TracingController).

Also fixed the problem of IO operations in UI thread.

BUG=none
TEST=TracingControllerTest.EnableAndDisableRecordingWithFilePath
TEST=TracingControllerTest.EnableCaptureAndDisableMonitoringWithFilePath
R=joi@chromium.org, nduca@chromium.org

Review URL: https://codereview.chromium.org/66893003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236071 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236071