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: I0664896bc335b1a757226aaa212536b8f9d0f08f
Reviewed-on: https://chromium-review.googlesource.com/293752Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>