memory coordinator: Add MemoryCoordinatorClient::OnPurgeMemory()

cc/tiles/software_image_decode_cache.cc

 // Copyright 2015 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/tiles/software_image_decode_cache.h"
 
 #include <inttypes.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1145 matching lines @@
         max_items_in_cache_ = kSuspendedMaxItemsInCache;
         break;
       case base::MemoryState::UNKNOWN:
         NOTREACHED();
         return;
     }
   }
   ReduceCacheUsage();
 }
 
+void SoftwareImageDecodeCache::OnPurgeMemory() {
+  // TODO(bashi): Make ReduceCacheUsage take a parameter and call
+  // ReduceCacheUsage(kSuspendedMaxItemsInCache). crbug.com/684287
+}
+
 }  // namespace cc