[Android] Fix crash if OmniboxStartupMetrics destroyed before delayed task is run

ToolbarManager#onDeferredStartup may post a delayed task to the UI thread that
calls itself. If the activity that owns the ToolbarManager is destroyed before
the task is run (and ToolbarManager#destroy is called) then the
mOmniboxStartupMetrics could be a null pointer. This cl adds a check to see if
the activity is finishing when the delayed task is run and noops out if it has.

Bug: 860449
Change-Id: Iac8333d26311142ce8e130b42aa5c7c551b844ca
Reviewed-on: https://chromium-review.googlesource.com/1127304Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573386}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/ToolbarManager.java

@@ -1223,6 +1223,7 @@ public class ToolbarManager implements ToolbarTabController, UrlFocusChangeListe
         long elapsedTime = SystemClock.elapsedRealtime() - activityCreationTimeMs;
         if (elapsedTime < RECORD_UMA_PERFORMANCE_METRICS_DELAY_MS) {
             ThreadUtils.postOnUiThreadDelayed(() -> {
+                if (mActivity.isActivityFinishing()) return;
                 onDeferredStartup(activityCreationTimeMs, activityName);
             }, RECORD_UMA_PERFORMANCE_METRICS_DELAY_MS - elapsedTime);
             return;