content: Cleanup - remove unused 'WebKit' forward declaration.

BUG=None
R=michaeln@chromium.org

Review URL: https://codereview.chromium.org/871243002

Cr-Commit-Position: refs/heads/master@{#313506}

content/renderer/service_worker/embedded_worker_dispatcher.h

@@ -13,10 +13,6 @@
 struct EmbeddedWorkerMsg_StartWorker_Params;
 class GURL;
 
-namespace WebKit {
-class WebEmbeddedWorker;
-}
-
 namespace content {
 
 // A tiny dispatcher which handles embedded worker start/stop messages.