events: Create a separate platforms target.

Split out the platform-event dispatch specific code out of the 'events'
target into its own 'events_platform' target, and the x11-specific code
into 'x11_events_platform' target.

BUG=361137
R=sky@chromium.org

Review URL: https://codereview.chromium.org/284743009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270816 0039d316-1c4b-4281-b951-d872f2087c98
12 files changed