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This adds a new IPC message QuitApp which is sent by the shim when the user
quits the shim (right click -> Quit). The AppShimHost responds by closing all
windows associated with the app. The shim actually quits when the extension
eventually closes and AppShimHost closes the channel to the shim.

BUG=168080
TEST=Start an app using its shim.
Right click the app in the dock and select quit.
The shell windows of that app should close.

Review URL: https://chromiumcodereview.appspot.com/14579005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203098 0039d316-1c4b-4281-b951-d872f2087c98