Evicted frames should immediately unblock dependent CompositorFrames

cc/surfaces/surface_dependency_tracker.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/surfaces/surface_dependency_tracker.h"
 
 #include "cc/surfaces/surface.h"
 #include "cc/surfaces/surface_info.h"
 #include "cc/surfaces/surface_manager.h"
 
@@ skipping 97 matching lines @@
         blocked_surfaces_.erase(surface_id);
     }
   }
 
   if (blocked_surfaces_.empty())
     frames_since_deadline_set_.reset();
 
   pending_surfaces_.erase(surface);
   surface->RemoveObserver(this);
 
-  // TODO(fsamuel): We should consider removing this surface as a dependency on
-  // other surfaces. This dependency will be removed when the deadline hits
-  // anyway but maybe we should try to actiate these frames early (see
-  // https://crbug.com/689725).
+  // Pretend that the discarded surface's SurfaceId is now available to unblock
+  // dependencies because we now know the surface will never activate.
+  NotifySurfaceIdAvailable(surface->surface_id());
 }
 
 void SurfaceDependencyTracker::OnSurfaceActivated(Surface* surface) {
   surface->RemoveObserver(this);
   pending_surfaces_.erase(surface);
   NotifySurfaceIdAvailable(surface->surface_id());
 }
 
 void SurfaceDependencyTracker::OnSurfaceDependenciesChanged(
     Surface* surface,
@@ skipping 42 matching lines @@
   // have a deadline.
   if (blocked_surfaces_.empty())
     frames_since_deadline_set_.reset();
 
   // Tell each surface about the availability of its blocker.
   for (Surface* blocked_pending_surface : blocked_pending_surface_set)
     blocked_pending_surface->NotifySurfaceIdAvailable(surface_id);
 }
 
 }  // namespace cc