Since chrome adds the ANGLE test source directly to it's own projects,
the main function is defined twice.  Remove the main file from the source
listings and only add it for our test executables.

BUG=angle:568

Change-Id: I30b481a23d5de80341d7f339583a43ad2f4391bb
Reviewed-on: https://chromium-review.googlesource.com/195345Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>