Fix -Wdefaulted-function-deleted warning

../../content/browser/renderer_host/render_widget_targeter.cc:63:22: warning:
explicitly defaulted move assignment operator is implicitly deleted
[-Wdefaulted-function-deleted]
  TracingUmaTracker& operator=(TracingUmaTracker&& tracker) = default;
                     ^
../../content/browser/renderer_host/render_widget_targeter.cc:73:13: note: move
assignment operator of 'TracingUmaTracker' is implicitly deleted because field
'id_' is of const-qualified type 'const int'
  const int id_;
            ^

Bug: 890307
Change-Id: I233c101f236053f02ec3affb9ea2b4655ad98d49
Reviewed-on: https://chromium-review.googlesource.com/c/1256922Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596140}

content/browser/renderer_host/render_widget_targeter.cc

@@ -60,7 +60,6 @@ class TracingUmaTracker {
   }
   ~TracingUmaTracker() = default;
   TracingUmaTracker(TracingUmaTracker&& tracker) = default;
-  TracingUmaTracker& operator=(TracingUmaTracker&& tracker) = default;
 
   void Stop() {
     TRACE_EVENT_ASYNC_END0(