Convert ExternalVkImageBacking representations

Convert Skia and GL representations for ExternalVkImageBacking to be
compatible to be able to handle multiple VkImages or GL textures. There
is no multiplanar support for dawn right now or overlay representations.

ExternalVkImageBacking still only contains a single Vulkan or GL texture
so this should be a no-op.

Bug: 1414560
Change-Id: I9e46290a4dd96ee23e4472c40d8c32fb232dd68b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4324477
Reviewed-by: Saifuddin Hitawala <hitawala@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1116545}