Allow the Mac theme provider to give default colors/tints if requested.

app/theme_provider.h

Index: app/theme_provider.h
===================================================================
--- app/theme_provider.h	(revision 39252)
+++ app/theme_provider.h	(working copy)
@@ -71,20 +71,23 @@
   // Gets the NSImage with the specified |id|.
   //
   // The bitmap is not assumed to exist. If a theme does not provide an image,
+  // if |allow_default| is true, then the default image will be returned, else
   // this function will return nil.
-  virtual NSImage* GetNSImageNamed(int id) const = 0;
+  virtual NSImage* GetNSImageNamed(int id, bool allow_default) const = 0;
 
   // Gets the NSColor with the specified |id|.
   //
-  // The color is not assumed to exist. If a theme does not provide an color,
-  // this function will return nil.
-  virtual NSColor* GetNSColor(int id) const = 0;
+  // The color is not assumed to exist. If a theme does not provide an color, if
+  // |allow_default| is true, then the default color will be returned, else this
+  // function will return nil.
+  virtual NSColor* GetNSColor(int id, bool allow_default) const = 0;
 
   // Gets the NSColor for tinting with the specified |id|.
   //
   // The tint is not assumed to exist. If a theme does not provide a tint with
-  // that id, this function will return nil.
-  virtual NSColor* GetNSColorTint(int id) const = 0;
+  // that id, if |allow_default| is true, then the default tint will be
+  // returned, else this function will return nil.
+  virtual NSColor* GetNSColorTint(int id, bool allow_default) const = 0;
 #elif defined(OS_POSIX) && !defined(TOOLKIT_VIEWS)
   // Gets the GdkPixbuf with the specified |id|.  Returns a pointer to a shared
   // instance of the GdkPixbuf.  This shared GdkPixbuf is owned by the theme