Mac Overlays: Allow multiple overlays with sandwich strategy

cc/output/overlay_strategy_common.h

Index: cc/output/overlay_strategy_common.h
diff --git a/cc/output/overlay_strategy_common.h b/cc/output/overlay_strategy_common.h
index 6d8c40559eb6afed356c8430ec9d0c73040fb75b..2593d51c1c67b3cac77ffa4f13e2d2f026f17429 100644
--- a/cc/output/overlay_strategy_common.h
+++ b/cc/output/overlay_strategy_common.h
@@ -19,12 +19,13 @@ class TextureDrawQuad;
 class CC_EXPORT OverlayStrategyCommonDelegate {
  public:
   virtual ~OverlayStrategyCommonDelegate() {}

[ccameron, 2015/09/02 21:41:55]

It might be easier to read to create an enum for the return type, something to
the effect of STOP_TRYING_OVERLAYS or KEEP_TRYING_OVERLAYS. And then pass the
the "iter" argument by pointer (continuing on the theme from ListContainer).

[Andre, 2015/09/02 21:54:51]

Do you still want this change if ListContainer::Erase returns an Iterator just
like InsertBefore?
I slightly prefer returning an Iterator. I think passing iter by pointer is less
clear at the call site that it's an input+output argument, especially with that
big list of arguments.

[ccameron, 2015/09/02 22:22:33]

I think there needs to be a clear "STOP_LOOKING_FOR_OVERLAYS versus
KEEP_LOOKING_FOR_OVERLAYS", and I think that the return value is the place for
it.

I think, actually, we need 3 return values
- CREATED_OVERLAY_STOP_LOOKING
  - in this case, we just return after TryOverlay
- CREATED_OVERLAY_KEEP_LOOKING
  - in this case, we keep calling TryOverlay. This requires that |iter| be
updated by TryOverlay (not too much to ask, because TryOverlay is already
already updated |candidate_list|
- DID_NOT_CREATE_OVERLAY
  - in this cause, we have to do ++iter

Blurgh, still not particularly clean.

[Andre, 2015/09/03 22:45:59]

Done.

-  virtual bool TryOverlay(OverlayCandidateValidator* capability_checker,
-                          RenderPassList* render_passes_in_draw_order,
-                          OverlayCandidateList* candidate_list,
-                          const OverlayCandidate& candidate,
-                          QuadList::Iterator iter,
-                          float device_scale_factor) = 0;
+  virtual QuadList::Iterator TryOverlay(
+      OverlayCandidateValidator* capability_checker,
+      RenderPassList* render_passes_in_draw_order,
+      OverlayCandidateList* candidate_list,
+      const OverlayCandidate& candidate,
+      QuadList::Iterator iter,
+      float device_scale_factor) = 0;
 };
 
 class CC_EXPORT OverlayStrategyCommon : public OverlayProcessor::Strategy {