media/gpu/V4L2SVDA: Make use of the GLFenceEGL wrapper.

Adapted the v4l2 video slice decoder to make use of the GLFenceEGL wrapper,
rather than using EGLSync functionality directly.

Benefits:
* Better code reuse and less code duplication.
* Makes it easier to mock out dependencies when unit testing.
* Easier to swap to different implementation if required.

TEST=ran VDA tests on kevin

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: If68ec97a92fba610762e475c108d0e6531c94be7
Reviewed-on: https://chromium-review.googlesource.com/1172176
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589071}