[Mac] Implement highlight-on-hover for the Wrench menu buttons.

chrome/browser/cocoa/menu_tracked_button.h

Index: chrome/browser/cocoa/menu_tracked_button.h
diff --git a/chrome/browser/cocoa/menu_tracked_button.h b/chrome/browser/cocoa/menu_tracked_button.h
index df42255fd9b94c828ed900f267feb30732c281ce..00dc78abe70287569cf1e56e9ba0934a825d83b8 100644
--- a/chrome/browser/cocoa/menu_tracked_button.h
+++ b/chrome/browser/cocoa/menu_tracked_button.h
@@ -12,6 +12,15 @@
 // the custom view of an NSMenuItem. If the user opens the menu in a non-sticky
 // fashion (i.e. clicks, holds, and drags) and then releases the mouse over
 // a MenuTrackedButton, it will |-performClick:| itself.
+//
+// To create the hover state effects, there are two code paths.  When the menu
+// is opened sticky, a tracking rect produces mouse entered/exit events that
+// allow for setting the cell's highlight property.  When in a drag cycle,
+// however, the only event received is |-mouseDragged:|.  Therefore, a
+// delayed selector is scheduled to poll the mouse location after each drag
+// event.  This checks if the user is still over the button after the drag
+// events stop being sent, indicating either the user is hovering without
+// movement or that the mouse is no longer over the receiver.
 @interface MenuTrackedButton : NSButton {
  @private
   // If the button received a |-mouseEntered:| event. This short-circuits the
@@ -21,6 +30,10 @@
   // Whether or not the user is in a click-drag-release event sequence. If so
   // and this receives a |-mouseUp:|, then this will click itself.
   BOOL tracking_;
+
+  // In order to get hover effects when the menu is sticky-opened, a tracking
+  // rect needs to be installed on the button.
+  NSTrackingRectTag trackingTag_;
 }
 
 @end