Refactored TransformFeedback for multi-platform ANGLE and added tests. 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/214105 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Kenneth Russell <kbr@chromium.org>