Removes shelling out to enumerate_files for the main targets (tests
and examples are currently unchanged) in favor of explicitly listing
out the files in the build.

There are a number of reasons. My immediate reason is that doing this
allows the file lists to be shared with the GN build so the GYP and GN
builds of angle should mostly automatically stay in sync now. In
addition, I don't think we should be enumerating files in GYP. No
other Chrome sub-projects do this, and Chrome engineers expect files
to be listed in .gyp* files. It's also slower to shell out a bunch of
times to get the file lists (this matters more in the GN build which
runs very quickly).

Updates the GN build to share these file lists. This unbreaks the
Windows GN Angle build.

BUG=angle:698

Change-Id: Iab8829628aeb7232bb64f4c247c35c6485bba3ee
Reviewed-on: https://chromium-review.googlesource.com/208461Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Brett Wilson <brettw@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>