[Android] Fix crash if ChromeTabbedActivity finishes early

A crash happens in ActivityTabStartupMetricsTracker if the tracked
ChromeActivity calls finish() but its onDestroy lifecycle method is not
called before chrome finishes its startup.
This cl adds a check to see if the ChromeActivity is finishing before
adding observers for metrics tracking.

Bug: 847580
Change-Id: I1a6685c770286c0533960c4ff7032dc047042c96
Reviewed-on: https://chromium-review.googlesource.com/1079202
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562925}

chrome/android/java/src/org/chromium/chrome/browser/metrics/ActivityTabStartupMetricsTracker.java

@@ -40,6 +40,10 @@ public class ActivityTabStartupMetricsTracker {
                 .addStartupCompletedObserver(new BrowserStartupController.StartupCallback() {
                     @Override
                     public void onSuccess() {
+                        // Activity could have called finish and returned early during startup but
+                        // not have onDestroy called yet. The activity's TabModelSelector may not
+                        // have been initialized causing a crash. See https://crbug.com/847580
+                        if (mActivity.isActivityFinishing()) return;
                         registerObservers();
                     }