[Sync] Implement Mac-specific UI for one-click signin

Enable one-click signin on OS X.

Build Cocoa version of one-click signin dialog.

Refactor one-click signin code to make it easier to unit test.

BUG=116685
TEST=On OSX, with a clean profile, sign into GMail.  The one-click infobar should pop up, and clicking "OK, sync" should bring up the one-click dialog.

Review URL: https://chromiumcodereview.appspot.com/9753003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@128126 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -372,9 +372,9 @@
           'webui_task_manager%': 1,   
         }],
 
-        # For now one-click signin is enabled only for windows since the UI
-        # is not yet complete for other platforms.
-        ['OS=="win"', {
+        # For now one-click signin is enabled only for windows and mac
+        # since the UI is not yet complete for other platforms.
+        ['OS=="win" or OS=="mac"', {
           'enable_one_click_signin%': 1,
         }],