blink MemoryCoordinator::onPurgeMemory should invoke each client's onPurgeMemory.

BUG=631822,670539

Review-Url: https://codereview.chromium.org/2732203002
Cr-Commit-Position: refs/heads/master@{#455990}

third_party/WebKit/Source/platform/MemoryCoordinator.cpp

@@ -68,6 +68,8 @@ void MemoryCoordinator::onMemoryStateChange(MemoryState state) {
 }
 
 void MemoryCoordinator::onPurgeMemory() {
+  for (auto& client : m_clients)
+    client->onPurgeMemory();
   // Don't call clearMemory() because font cache invalidation always causes full
   // layout. This increases tab switching cost significantly (e.g.
   // en.wikipedia.org/wiki/Wikipedia). So we should not invalidate the font