memory coordinator: Add MemoryCoordinatorClient::OnPurgeMemory()

cc/tiles/software_image_decode_cache.cc

Index: cc/tiles/software_image_decode_cache.cc
diff --git a/cc/tiles/software_image_decode_cache.cc b/cc/tiles/software_image_decode_cache.cc
index aa2066592517f744eaaaa1020d2ce9bf5d26c61d..2628cd96b06e0a7012a60b7994e74d452e61bc51 100644
--- a/cc/tiles/software_image_decode_cache.cc
+++ b/cc/tiles/software_image_decode_cache.cc
@@ -1163,4 +1163,9 @@ void SoftwareImageDecodeCache::OnMemoryStateChange(base::MemoryState state) {
   ReduceCacheUsage();
 }
 
+void SoftwareImageDecodeCache::OnPurgeMemory() {
+  // TODO(bashi): Make ReduceCacheUsage take a parameter and call
+  // ReduceCacheUsage(kSuspendedMaxItemsInCache). crbug.com/684287
+}
+
 }  // namespace cc