Implement Native EventTimeFromNative on Mac
Implement Native EventTimeFromNative on Mac

Implements Mac version of new |EventTimeFromNative| function.  Introduced
in r117907 on other platforms.

BUG=109946
TEST=Manual testing with Aura Shell on Mac
R=sky@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9242019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@117978 0039d316-1c4b-4281-b951-d872f2087c98

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