Fix the Mac debug build

event.cc was added to the build to get ui::TouchEvent and ui::GestureEvent
but this file is not built on Mac by Chromium itself and thus fails to build.

Change-Id: Ib7bb4fb1fc8692df1b8151c3d159de3d0f73c76f
Reviewed-by: Andras Becsi <andras.becsi@digia.com>