Fix -Wdefaulted-function-deleted warning

This is part of cleaning up for a new Clang warning that's currently disabled:

In file included from ../../ash/app_list/home_launcher_gesture_handler.cc:19:
../../ash/wm/window_transient_descendant_iterator.h:69:43: error: explicitly
defaulted copy assignment operator is implicitly deleted
[-Werror,-Wdefaulted-function-deleted]
  WindowTransientDescendantIteratorRange& operator=(
                                          ^
../../ash/wm/window_transient_descendant_iterator.h:72:37: note: copy
assignment operator of 'WindowTransientDescendantIteratorRange' is implicitly
deleted because field 'begin_' has an inaccessible copy assignment operator
  WindowTransientDescendantIterator begin_;
                                    ^

Bug: 890307
Change-Id: Ic47bc989f9fe26e1387f9a6a8c00f19fcf6b91ce
Reviewed-on: https://chromium-review.googlesource.com/c/1340331
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609220}

ash/wm/window_transient_descendant_iterator.h

@@ -64,10 +64,10 @@ class WindowTransientDescendantIteratorRange {
   const WindowTransientDescendantIterator& end() const { return end_; }
 
  private:
-  // Explicit assignment operator defined because an explicit copy constructor
-  // is needed and therefore the DISALLOW_COPY_AND_ASSIGN macro cannot be used.
+  // Because the explicit copy constructor is needed, explicitly delete the
+  // assignment operator rather than using DISALLOW_COPY_AND_ASSIGN.
   WindowTransientDescendantIteratorRange& operator=(
-      const WindowTransientDescendantIteratorRange& other) = default;
+      const WindowTransientDescendantIteratorRange& other) = delete;
 
   WindowTransientDescendantIterator begin_;
   WindowTransientDescendantIterator end_;