content: Cleanup class/struct forward declarations

content/browser/renderer_host/render_process_host_impl.h

Index: content/browser/renderer_host/render_process_host_impl.h
diff --git a/content/browser/renderer_host/render_process_host_impl.h b/content/browser/renderer_host/render_process_host_impl.h
index cf4139f6979b42523cafad16a1e94a48f3b74b0f..93c9fd00a34af0863dd0e57dcae89ad95511755d 100644
--- a/content/browser/renderer_host/render_process_host_impl.h
+++ b/content/browser/renderer_host/render_process_host_impl.h
@@ -48,14 +48,6 @@ class MessageLoop;
 class SharedPersistentMemoryAllocator;
 }
 
-namespace gfx {
-class Size;
-}
-
-namespace IPC {
-class ChannelMojoHost;
-}
-
 namespace content {
 class AudioInputRendererHost;
 class AudioRendererHost;
@@ -72,8 +64,6 @@ class RendererMainThread;
 class RenderFrameMessageFilter;
 class RenderWidgetHelper;
 class RenderWidgetHost;
-class RenderWidgetHostImpl;
-class RenderWidgetHostViewFrameSubscriber;
 class ResourceMessageFilter;
 class StoragePartition;
 class StoragePartitionImpl;