viz: Enable SA partial output for SkiaRenderer

SurfaceAggregator has an option to output only quads that
intersect the damage rect computed by SA itself.

crrev.com/c/2205886 re-enabled a SurfaceAggregator optimization for
GLRenderer but left it disabled for SkiaRenderer/

This patch enables the optimization for SkiaRenderer.

Bug: 925181
Change-Id: I1ad90ab2ec0d3a5b5444350ffadb7c3a5f2469ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250780Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779836}

components/viz/service/display_embedder/skia_output_surface_impl.cc

@@ -702,6 +702,7 @@ bool SkiaOutputSurfaceImpl::Initialize() {
   if (capabilities_.preserve_buffer_content &&
       capabilities_.supports_post_sub_buffer) {
     capabilities_.only_invalidates_damage_rect = false;
+    capabilities_.supports_target_damage = true;
     damage_of_buffers_.resize(capabilities_.max_frames_pending + 1);
   }