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This is third CL for adding an item for dialog.
This cl adds new LauncherItemType, TYPE_DIALOG, and 
implements a context menu of item created by ShelfWindowWatcher.

R=sky@chromium.org
BUG=121242
TEST=NONE

Review URL: https://codereview.chromium.org/107163005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243965 0039d316-1c4b-4281-b951-d872f2087c98