Fix building on the 10.7 SDK.

NSEventPhaseMayBegin showed up in the 10.8 SDK, so make sure that if we're building with anything before the 10.8 SDK, we define it.

BUG=138175
TEST=the Mac 64 builder goes green.
TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/107163007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242420 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242420