Cache Storage: Add Platform API to retrieve CacheStorage for origin

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index 6f50d496c3e41f01c650a76c141327faa9991f22..b592c49e10b6a7170b7d0a02a17d8d6a19e28bc0 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -101,6 +101,7 @@ class WebSandboxSupport;
 class WebScheduler;
 class WebSecurityOrigin;
 class WebScrollbarBehavior;
+class WebServiceWorkerCacheStorage;
 class WebSocketHandle;
 class WebSpeechSynthesizer;
 class WebSpeechSynthesizerClient;
@@ -219,6 +220,11 @@ public:
     virtual WebIDBFactory* idbFactory() { return 0; }
 
 
+    // Cache Storage ----------------------------------------------------------
+
+    // The caller is responsible for deleting the returned object.
+    virtual WebServiceWorkerCacheStorage* cacheStorage(const WebString& originIdentifier) { return nullptr; }
+
     // Gamepad -------------------------------------------------------------
 
     virtual void sampleGamepads(WebGamepads& into) { into.length = 0; }