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AppShimHost is now confined to interacting with the app shim and passing
events to AppShimHandlers. ExtensionAppShimHandler becomes the default
handler for app shim events.

BUG=168080
TEST=There should be no behavior change with this refactor.

Review URL: https://chromiumcodereview.appspot.com/15269003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202791 0039d316-1c4b-4281-b951-d872f2087c98