Don't include abort() cost when timing MSE methods.

xhr.abort() and sourceBuffer.abort() were transitively included
via onPlaybackStarted().  It's not relevant to the recorded
metrics so measure end time before the onPlaybackStarted()
calls.

BUG=367228
TEST=ran telemetry tests.

Review URL: https://codereview.chromium.org/251593008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266378 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/page_sets/mse_cases/startup_test.js

@@ -375,12 +375,11 @@
         return;
       }
 
+      var testEndTime = getPerfTimestamp();
       for (var i = 0; i < appenders.length; ++i) {
         appenders[i].onPlaybackStarted(mediaSource);
       }
 
-      var testEndTime = getPerfTimestamp();
-
       testDone = true;
       window.clearInterval(listener);
       window.clearTimeout(timeout);