Destroy activityTabObserver in ToolbarManager

Without destroying this observer, ToolbarManager will still receive and respond to tab change notifications, which can in theory happen post-destruction. This change also adds a null guard in OptionalBrowsingModeButtonController.

Bug: 1071254
Change-Id: I5730f1b173dd0ff803fab0350b56513df69b5eaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153687Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759846}

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

@@ -1127,6 +1127,12 @@ public class ToolbarManager implements ToolbarTabController, UrlFocusChangeListe
 
         if (mTabThemeColorProvider != null) mTabThemeColorProvider.removeThemeColorObserver(this);
         if (mAppThemeColorProvider != null) mAppThemeColorProvider.destroy();
+
+        if (mActivityTabTabObserver != null) {
+            mActivityTabTabObserver.destroy();
+            mActivityTabTabObserver = null;
+        }
+
         mActivity.unregisterComponentCallbacks(mComponentCallbacks);
         mComponentCallbacks = null;
         AccessibilityUtil.removeObserver(this);

chrome/android/java/src/org/chromium/chrome/browser/toolbar/top/OptionalBrowsingModeButtonController.java

@@ -85,6 +85,7 @@ class OptionalBrowsingModeButtonController {
      * Show the highest precedence optional button, hiding the optional button if none can be shown.
      */
     private void showHighestPrecedenceOptionalButton() {
+        if (mButtonDataProviders == null) return;
         for (ButtonDataProvider provider : mButtonDataProviders) {
             ButtonData buttonData = provider.get(mTabSupplier.get());
             if (buttonData != null && buttonData.canShow) {