Mac Overlays: Allow multiple overlays with sandwich strategy

cc/output/overlay_strategy_common.cc

Index: cc/output/overlay_strategy_common.cc
diff --git a/cc/output/overlay_strategy_common.cc b/cc/output/overlay_strategy_common.cc
index 5d79cd09878555a375dd715f0421b92079738617..dfad68c1edb6dc4470c42b8eafc40240dd6837c4 100644
--- a/cc/output/overlay_strategy_common.cc
+++ b/cc/output/overlay_strategy_common.cc
@@ -33,18 +33,39 @@ bool OverlayStrategyCommon::Attempt(RenderPassList* render_passes_in_draw_order,
   RenderPass* root_render_pass = render_passes_in_draw_order->back();
   DCHECK(root_render_pass);
 
+  // Add our primary surface.
+  OverlayCandidate main_image;
+  main_image.display_rect = root_render_pass->output_rect;
+  DCHECK(candidate_list->empty());
+  candidate_list->push_back(main_image);
+
+  bool success = false;
   QuadList& quad_list = root_render_pass->quad_list;
-  for (auto it = quad_list.begin(); it != quad_list.end(); ++it) {
+  for (auto it = quad_list.begin(); it != quad_list.end();) {
     OverlayCandidate candidate;
     const DrawQuad* draw_quad = *it;
     if (IsOverlayQuad(draw_quad) &&
-        GetCandidateQuadInfo(*draw_quad, &candidate) &&
-        delegate_->TryOverlay(capability_checker_, render_passes_in_draw_order,
-                              candidate_list, candidate, it,
-                              device_scale_factor))
-      return true;
+        GetCandidateQuadInfo(*draw_quad, &candidate)) {
+      OverlayResult result = delegate_->TryOverlay(
+          capability_checker_, render_passes_in_draw_order, candidate_list,
+          candidate, &it, device_scale_factor);
+      if (result == CREATED_OVERLAY_STOP_LOOKING) {

[alexst (slow to review), 2015/09/04 17:53:57]

Actually, one more thought, we don't handle DID_NOT_CREATE_OVERLAY anywhere, I
know it's preferred to handle all enum cases so they don't accidentally fall
through the cracks and result in undefined behaviour. What do you think of a
switch statement instead?

[Andre, 2015/09/08 16:55:36]

Done.
The combination of continue (for the loop) and break (for the switch) didn't
look great, so I did a little refactor to improve it a bit.
PTAL.

+        success = true;
+        break;
+      } else if (result == CREATED_OVERLAY_KEEP_LOOKING) {
+        success = true;
+        continue;
+      }
+    }
+    ++it;
   }
-  return false;
+
+  if (!success) {
+    DCHECK_EQ(1u, candidate_list->size());
+    candidate_list->clear();
+  }
+
+  return success;
 }
 
 bool OverlayStrategyCommon::IsOverlayQuad(const DrawQuad* draw_quad) {