Bug 1578042 - Add a CompositionOpportunityId counter and implement it for WebRender. r=nical

This ID is different from the IDs we already have:
 - It is different from VsyncId because it doesn't skip numbers when composites
   are delayed.
 - It is different from RenderedFrameId because it also increases for no-op
   composites.
 - It is different from transaction IDs and epochs because it doesn't care about
   the content side, it just looks at compositing.

It is currently not implemented for non-WR. In non-WR the video frame drop
detection wouldn't properly work anyway, because UpdateCompositedFrame is not
called on every composite for non-WR.

Differential Revision: https://phabricator.services.mozilla.com/D83461

gfx/layers/LayersTypes.h

@@ -151,6 +151,20 @@ struct LayersObserverEpoch {
   }
 };
 
+// CompositionOpportunityId is a counter that goes up every time we have an
+// opportunity to composite. It increments even on no-op composites (if nothing
+// has changed) and while compositing is paused. It does not skip values if a
+// composite is delayed. It is meaningful per window.
+// This counter is used to differentiate intentionally-skipped video frames from
+// unintentionally-skipped video frames: If CompositionOpportunityIds are
+// observed by the video in +1 increments, then the video was onscreen the
+// entire time and compositing was not paused. But if gaps in
+// CompositionOpportunityIds are observed, that must mean that the video was not
+// considered during some composition opportunities, because compositing was
+// paused or because the video was not part of the on-screen scene.
+class CompositionOpportunityType {};
+typedef BaseTransactionId<CompositionOpportunityType> CompositionOpportunityId;
+
 enum class LayersBackend : int8_t {
   LAYERS_NONE = 0,
   LAYERS_BASIC,

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -1960,6 +1960,7 @@ void WebRenderBridgeParent::CompositeToTarget(VsyncId aId,
   AUTO_PROFILER_TRACING_MARKER("Paint", "CompositeToTarget", GRAPHICS);
   if (mPaused || !mReceivedDisplayList) {
     ResetPreviousSampleTime();
+    mCompositionOpportunityId = mCompositionOpportunityId.Next();
     TimeStamp now = TimeStamp::Now();
     PROFILER_ADD_TEXT_MARKER("SkippedComposite",
                              mPaused ? "Paused"_ns : "No display list"_ns,
@@ -1987,6 +1988,8 @@ void WebRenderBridgeParent::CompositeToTarget(VsyncId aId,
                              JS::ProfilingCategoryPair::GRAPHICS, now, now);
     return;
   }
+
+  mCompositionOpportunityId = mCompositionOpportunityId.Next();
   MaybeGenerateFrame(aId, /* aForceGenerateFrame */ false);
 }
 

gfx/layers/wr/WebRenderBridgeParent.h

@@ -506,6 +506,7 @@ class WebRenderBridgeParent final
   std::queue<CompositorAnimationIdsForEpoch> mCompositorAnimationsToDelete;
   wr::Epoch mWrEpoch;
   wr::IdNamespace mIdNamespace;
+  CompositionOpportunityId mCompositionOpportunityId;
 
   VsyncId mSkippedCompositeId;
   TimeStamp mMostRecentComposite;