Cache Storage: Add Platform API to retrieve CacheStorage for origin

public/web/WebServiceWorkerContextClient.h

Index: public/web/WebServiceWorkerContextClient.h
diff --git a/public/web/WebServiceWorkerContextClient.h b/public/web/WebServiceWorkerContextClient.h
index a3619d5b4cfbd66cdcfd893dcefd1e62baacfccd..2759bee3c7368b7a337dba70ef8f6711f7c72f28 100644
--- a/public/web/WebServiceWorkerContextClient.h
+++ b/public/web/WebServiceWorkerContextClient.h
@@ -43,7 +43,6 @@ namespace blink {
 struct WebCrossOriginServiceWorkerClient;
 struct WebServiceWorkerClientQueryOptions;
 class WebDataSource;
-class WebServiceWorkerCacheStorage;
 class WebServiceWorkerContextProxy;
 class WebServiceWorkerNetworkProvider;
 class WebServiceWorkerProvider;
@@ -62,9 +61,6 @@ class WebServiceWorkerContextClient {
 public:
     virtual ~WebServiceWorkerContextClient() { }
 
-    // ServiceWorker specific method.
-    virtual WebServiceWorkerCacheStorage* cacheStorage() { return nullptr; }
-
     // ServiceWorker specific method. Called when script accesses the
     // the |scope| attribute of the ServiceWorkerGlobalScope. Immutable per spec.
     virtual WebURL scope() const { return WebURL(); }