Reland with a fix for SurfaceTest in angle_unittests and fixes for
signed-unsigned warnings

In CGL there is no notion of default Framebuffer and MakeCurrent only
makes a context current but not a drawable. Instead, everything is done
via render to texture. For that reason, different surfaces will have
different FBOs as default framebuffers, which causes that change.

BUG=angleproject:891

Change-Id: Ie9a72de01a58e583a1bfa3f4a055debb860b912f
Reviewed-on: https://chromium-review.googlesource.com/293713Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>