Bug fix: Combobox: Focus on clicking mouse button

This CL fixes the regression that Combobox doesn't get focused when
clicking a right button of a mouse. Before introducing
TransparentButton, Combobox can get focus by itself on the mouse button
events, but now, TranparentButton robs the combobox of the events.
This CL has TransparentButton::OnMousePressed help the combobox with
focusing it.

BUG=350811
TEST=manual

Review URL: https://codereview.chromium.org/189913014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256016 0039d316-1c4b-4281-b951-d872f2087c98

ui/views/controls/combobox/combobox.cc

@@ -91,6 +91,11 @@ class TransparentButton : public CustomButton {
   }
   virtual ~TransparentButton() {}
 
+  virtual bool OnMousePressed(const ui::MouseEvent& mouse_event) OVERRIDE {
+    parent()->RequestFocus();
+    return true;
+  }
+
   double GetAnimationValue() const {
     return hover_animation_->GetCurrentValue();
   }