[Android] Support startup histogram.
This CL adds support for the
Startup.BrowserMessageLoopStartTimeFromMainEntry UMA histogram to
chromium_testshell. It does this by recording the start time as
early as possible on the Java side (well before the C++ library is even
loaded), and then reading this on the C++ side through a new JNI call.
BUG=266173
Review URL: https://chromiumcodereview.appspot.com/
23443009
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@220588 0039d316-1c4b-4281-b951-d872f2087c98