Mojo: Add a basic multiprocess MessagePipe test.

Not all the machinery is there yet, and we can't yet pass handles, but
it's a start.

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/99323008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240294 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240294
Make LatencyInfo trace show input event name

It is useful to know the event type of the LatencyInfo when inspecting trace.

BUG=246034
TEST=When tracing input event latency, one can see the event type of the LatencyInfo trace.

Review URL: https://codereview.chromium.org/103863008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240295 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240295