lacros: Mark gfx.mojom.ImageSkia as [Stable]

It is unchanged since it was refactored to use skia.mojom.Bitmap in
early 2018. I recently marked skia.mojom.Bitmap as [Stable], so its
only dependency is [Stable].

Last non-trivial change:
https://chromium-review.googlesource.com/c/chromium/src/+/1004598/

This will allow me to use it in Lacros for the notifications API, to
transport high-DPI icon images.

Bug: 1116652
Test: presubmits
Change-Id: I7161ee8666483f8b536c390d50e9e5cf54872ecc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386458Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803541}

ui/gfx/image/mojom/image.mojom

@@ -6,6 +6,7 @@ module gfx.mojom;
 
 import "skia/public/mojom/bitmap.mojom";
 
+[Stable]
 struct ImageSkiaRep {
   // Transport of the bitmap in this representation.
   skia.mojom.Bitmap bitmap;
@@ -17,6 +18,7 @@ struct ImageSkiaRep {
 // Mojo transport for an ImageSkia via shared buffer. Note that transporting an
 // ImageSkia over mojo will load all of its image representations for supported
 // scales.
+[Stable]
 struct ImageSkia {
   array<ImageSkiaRep> image_reps;
 };