[Android] Fix crash if browser startup completes before activity

Currently a crash can occur if CustomTabsConnections starts the browser
synchronously while an async browser startup is in progress.

Bug: 881019
Change-Id: I2d1b764c8212a05b85f59c5edfa92ac28a4171f9
Reviewed-on: https://chromium-review.googlesource.com/1210623Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589204}

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

@@ -40,10 +40,9 @@ 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;
+                        // The activity's TabModelSelector may not have been initialized yet
+                        // causing a crash. See https://crbug.com/847580
+                        if (!mActivity.areTabModelsInitialized()) return;
                         registerObservers();
                     }