Add instance & device extensions provided by layers prior to running
VerifyExtensionsPresent() so that we won't abort when required extensions
are provided in a layer. This happens particularly on Fuchsia as the
swapchain extension is implemented in a layer.

BUG=angleproject:2475
TEST=angle_end2end_tests on Fuchsia

Change-Id: I30385dfe55b14783a20d1410f1209d3e3ad10632
Reviewed-on: https://chromium-review.googlesource.com/c/1456487Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Michael Spang <spang@chromium.org>