Added angle_implementation_unit_tests target designed for testing the
cross-platform code in libGLESv2, and libGLESv2_static target as a
dependency. The goal is to incorporate these tests into Chromium's
angle_unittests target on all platforms; however, more work is needed
to make libGLESv2's common code compile on non-Windows platforms, so
this is an intermediate step.

BUG=angle:719

Change-Id: Ifc44a779352294e857d6e913d9b997a60674c443
Reviewed-on: https://chromium-review.googlesource.com/214105Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>