Add WidgetObserver methods for drag start/end.

These will be used by test code to trigger events based on dragging.  Tests
can't reliably e.g. "queue event to trigger a drag and then call a particular
callback" due to the nested, filtered message loop on Windows.  Using drag
observer methods is reliable, and clearer as well.

Bug: 923188
Change-Id: Id24b8150d04dc878dce543fe469994aba29694e4
Reviewed-on: https://chromium-review.googlesource.com/c/1483931
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634869}

ui/views/widget/widget.cc

@@ -804,6 +804,9 @@ void Widget::RunShellDrag(View* view,
   dragged_view_ = view;
   OnDragWillStart();
 
+  for (WidgetObserver& observer : observers_)
+    observer.OnWidgetDragWillStart(this);
+
   WidgetDeletionObserver widget_deletion_observer(this);
   native_widget_->RunShellDrag(view, data, location, operation, source);
 
@@ -818,6 +821,9 @@ void Widget::RunShellDrag(View* view,
     view->OnDragDone();
   }
   OnDragComplete();
+
+  for (WidgetObserver& observer : observers_)
+    observer.OnWidgetDragComplete(this);
 }
 
 void Widget::SchedulePaintInRect(const gfx::Rect& rect) {

ui/views/widget/widget_observer.h

@@ -37,8 +37,11 @@ class VIEWS_EXPORT WidgetObserver : public base::CheckedObserver {
   // widget has been destroyed.
   virtual void OnWidgetDestroyed(Widget* widget) {}
 
-  virtual void OnWidgetVisibilityChanging(Widget* widget, bool visible) {}
+  // Called before RunShellDrag() is called and after it returns.
+  virtual void OnWidgetDragWillStart(Widget* widget) {}
+  virtual void OnWidgetDragComplete(Widget* widget) {}
 
+  virtual void OnWidgetVisibilityChanging(Widget* widget, bool visible) {}
   virtual void OnWidgetVisibilityChanged(Widget* widget, bool visible) {}
 
   virtual void OnWidgetActivationChanged(Widget* widget, bool active) {}