Add null check to BottomToolbarMediator#destroy()

If the BottomToolbar is destroyed before native is initialized there
will be a null pointer exception. This CL adds a null check to
prevent this.

Bug: 854342
Change-Id: Ic0a82574e87bce7f46b20d3621796fad09c119a5
Reviewed-on: https://chromium-review.googlesource.com/1107053
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569407}

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

@@ -72,8 +72,8 @@ class BottomToolbarMediator
      */
     public void destroy() {
         mFullscreenManager.removeListener(this);
-        mOverviewModeBehavior.removeOverviewModeObserver(this);
         if (mContextualSearchManger != null) mContextualSearchManger.removeObserver(this);
+        if (mOverviewModeBehavior != null) mOverviewModeBehavior.removeOverviewModeObserver(this);
     }
 
     @Override