Decrement pending readbacks with skia

SkiaOutputSurfaceImplOnGpu::num_readbacks_pending_ isn't decremented
when readback is complete. CheckReadbackCompletion() will continue to
run periodically even when there are no pending readbacks as a result.

Bug: 1022495
Change-Id: I636d83db87bcad0debbf3098d25023d88e5947b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993392
Auto-Submit: kylechar <kylechar@chromium.org>
Commit-Queue: Jonathan Backer <backer@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729880}

components/viz/service/display_embedder/skia_output_surface_impl_on_gpu.cc

@@ -222,6 +222,9 @@ void OnYUVReadbackDone(
     std::unique_ptr<const SkSurface::AsyncReadResult> async_result) {
   std::unique_ptr<ReadPixelsContext> context(
       static_cast<ReadPixelsContext*>(c));
+  if (context->impl_on_gpu)
+    context->impl_on_gpu->ReadbackDone();
+
   if (!async_result) {
     // This will automatically send an empty result.
     return;
@@ -243,6 +246,9 @@ void OnRGBAReadbackDone(
     std::unique_ptr<const SkSurface::AsyncReadResult> async_result) {
   std::unique_ptr<ReadPixelsContext> context(
       static_cast<ReadPixelsContext*>(c));
+  if (context->impl_on_gpu)
+    context->impl_on_gpu->ReadbackDone();
+
   if (!async_result) {
     // This will automatically send an empty result.
     return;