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Since hiding Chrome hides all apps, this informs the shims that their
app is hidden. This ensures that a unhide is fired when the shim is next
focused.

BUG=355397

Review URL: https://codereview.chromium.org/403743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284622 0039d316-1c4b-4281-b951-d872f2087c98