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Currently we only bounce the app shim when the app is hidden but wants
to show a window.

This CL makes AppWindow.drawAttention() bounce the app icon continuously
until the app is focused or AppWindow.clearAttention() is called.

BUG=402722

Review URL: https://codereview.chromium.org/461303002

Cr-Commit-Position: refs/heads/master@{#290768}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290768 0039d316-1c4b-4281-b951-d872f2087c98