libchrome: Fix the build in OS X > 10.10

For some reason, there is a conflict in the forward declaration of
NSUserActivity. This allows newer versions of OS X than what the
buildbots have to be able to build libchrome.

Bug: 28985443
Change-Id: I3ba3af04fcaba086862f3114250324348c0c9cf2
Test: mmma -j external/libchrome # in El Capitan
1 file changed