mac: Allow users to dual-wield magic trackpad/mouse.

Using the api `[NSEvent trackSwipeEventWithOptions:]` blocks NSTouch callbacks
for a non-deterministic amount of time. Once we activate the API once, never
try to use NSTouches on the same thread again.

For the user, the functional effect is that if they ever uses a magic mouse to
initiate a swipe, we disable all the new magic trackpad logic (vertical swiping
dismisses arrow), and revert to previous behavior (vertical swiping is
ignored).

BUG=328583
TEST=See repro steps in bug. Magic trackpad history swiping should always work,
regardless of whether a user has ever initiated a magic mouse history swipe.
(The new vertical swipe to dismiss arrow functionality will be lost).

Review URL: https://codereview.chromium.org/112433005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242948 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242948