This changes angle_unittests.gypi to use a variable per list of files so
that we can grab the list of files in conditions in BUILD.gn

BUG=angleproject:929

Change-Id: I7fc73fd84d8efc1ea545fe672b1dba1c429b317f
Reviewed-on: https://chromium-review.googlesource.com/296443Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>