memory coordinator: Add MemoryCoordinatorClient::OnPurgeMemory()

cc/tiles/gpu_image_decode_cache.cc

Index: cc/tiles/gpu_image_decode_cache.cc
diff --git a/cc/tiles/gpu_image_decode_cache.cc b/cc/tiles/gpu_image_decode_cache.cc
index 7f75c888d8adc413831e88726b609c0cd00a0d1e..d5418a3cbc71300ef0de0ec09aa678af4e43c7af 100644
--- a/cc/tiles/gpu_image_decode_cache.cc
+++ b/cc/tiles/gpu_image_decode_cache.cc
@@ -1261,24 +1261,14 @@ bool GpuImageDecodeCache::DiscardableIsLockedForTesting(
 }
 
 void GpuImageDecodeCache::OnMemoryStateChange(base::MemoryState state) {
-  switch (state) {
-    case base::MemoryState::NORMAL:
-      memory_state_ = state;
-      break;
-    case base::MemoryState::THROTTLED:
-    case base::MemoryState::SUSPENDED: {
-      memory_state_ = state;
-
-      // We've just changed our memory state to a (potentially) more
-      // restrictive one. Re-enforce cache limits.
-      base::AutoLock lock(lock_);
-      EnsureCapacity(0);
-      break;
-    }
-    case base::MemoryState::UNKNOWN:
-      // NOT_REACHED.
-      break;
-  }
+  memory_state_ = state;
+}
+
+void GpuImageDecodeCache::OnPurgeMemory() {
+  // We've just changed our memory state to a (potentially) more
+  // restrictive one. Re-enforce cache limits.
+  base::AutoLock lock(lock_);
+  EnsureCapacity(0);
 }
 
 }  // namespace cc