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- Create ShellAcceleratorController that manages global keyboard accelerators and also processes several accelerators as a target.
- Create ShellAcceleratorFilter, which is used by DesktopEventFilter to handle accelerators.
- Add a function to Shell for accessing ShellAcceleratorController.

The 1st attempt (http://crrev.com/110637) broke aura_shell_unittests and this fixes it.
The differences are
- moving ShellAcceleratorFilter to Shell from DesktopEventFilter, and
- adding the ShellAcceleratorFilter in Shell initialization code to DesktopEventFilter.

BUG=97255
TEST=Manual, aura_shell_unittests succeeds.


Review URL: http://codereview.chromium.org/8561012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111100 0039d316-1c4b-4281-b951-d872f2087c98