Implement OnMemoryStateChange for Various CC Classes

cc/resources/resource_pool.cc

 // Copyright 2012 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/resources/resource_pool.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 451 matching lines @@
 void ResourcePool::OnMemoryStateChange(base::MemoryState state) {
   switch (state) {
     case base::MemoryState::NORMAL:
       // TODO(tasak): go back to normal state.
       break;
     case base::MemoryState::THROTTLED:
       // TODO(tasak): make the limits of this component's caches smaller to
       // save memory usage.
       break;
     case base::MemoryState::SUSPENDED:
-      // TODO(tasak): free this component's caches as much as possible before
-      // suspending renderer.
+      // We are only ever SUSPENDED while in the background. While in the
+      // background, ResourcePool already has zero memory limits and should not
+      // be holding any resources that aren't actively in use.
+      DCHECK_EQ(0, unused_resources_.size());

[vmpstr, 2016/09/23 20:59:10]

Are we guaranteed that we receive this signal after we get backgrounded? I'm
curious about order of events here.

[ericrk, 2016/09/23 21:26:15]

My understanding was that we are only suspended while invisible. That said, it
seems fragile to rely on this ordering. Re-structed so that we can handle any
order of calls.

+
+      // The following call will drop additional busy resources. There is no
+      // negative to this other than temporary incorrect accounting, and it
+      // ensures the resource is freed as soon as possible.
+      EvictResourcesNotUsedSince(base::TimeTicks() + base::TimeDelta::Max());
       break;
     case base::MemoryState::UNKNOWN:
       // NOT_REACHED.
       break;
   }
 }
 
 }  // namespace cc