[Mac] First pass at multiple selection in the tabstrip.

This hooks up a new TabController state (called 'active') to indicate what
'selected' used to: key/focused state. The selected state can now be applied to
multiple TabControllers.

Currently only commands available through the context menu are usable for a
multi-tab selection. Support for dragging will come in a future CL.

The other notable chane is that tab activation now happens on |-mouseUp:|
rather than |-mouseDown:|, which is in preparation for multi-tab dragging.

BUG=78816
TEST=Shift click, Cmd click, and Cmd+Shift click to select tabs in different ways.

Review URL: http://codereview.chromium.org/6821003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@82543 0039d316-1c4b-4281-b951-d872f2087c98
16 files changed