Fix segfault in VR with Top Document Isolation on.

The touch selection controller may be null on Android for a few reasons,
one of which is that selection is disabled in VR currently.

Bug: 822374
Change-Id: Ie19fc8a4b148f64dc0fda391a45e758940d39154
Reviewed-on: https://chromium-review.googlesource.com/963680Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543726}

content/browser/renderer_host/input/touch_selection_controller_client_manager_android.cc

@@ -71,8 +71,10 @@ void TouchSelectionControllerClientManagerAndroid::UpdateClientSelectionBounds(
   // Notify TouchSelectionController if anything should change here. Only
   // update if the client is different and not making a change to empty, or
   // is the same client.
+  if (GetTouchSelectionController()) {
     GetTouchSelectionController()->OnSelectionBoundsChanged(
         manager_selection_start_, manager_selection_end_);
+  }
 }
 
 void TouchSelectionControllerClientManagerAndroid::InvalidateClient(

content/public/browser/touch_selection_controller_client_manager.h

@@ -55,7 +55,7 @@ class CONTENT_EXPORT TouchSelectionControllerClientManager {
   virtual void InvalidateClient(ui::TouchSelectionControllerClient* client) = 0;
 
   // Provides direct access to the TouchSelectionController that will be used
-  // with all clients accessing this manager. Always returns non-null value.
+  // with all clients accessing this manager. May return null values on Android.
   virtual ui::TouchSelectionController* GetTouchSelectionController() = 0;
 
   // The following two functions allow clients (or their owners, etc.) to