Enable spare RenderProcessHost to be preinitialized.

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 96900941e917bf964a13293930b129bb36f9ab78..8a500b1fda41856722ecd5447ddd9b8e2e60d928 100644
--- a/content/browser/renderer_host/render_process_host_impl.h
+++ b/content/browser/renderer_host/render_process_host_impl.h
@@ -114,9 +114,11 @@ class CONTENT_EXPORT RenderProcessHostImpl
       public NON_EXPORTED_BASE(mojom::AssociatedInterfaceProvider),
       public NON_EXPORTED_BASE(mojom::RendererHost) {
  public:
-  RenderProcessHostImpl(BrowserContext* browser_context,
-                        StoragePartitionImpl* storage_partition_impl,
-                        bool is_for_guests_only);
+  static RenderProcessHost* CreateRenderProcessHost(
+      BrowserContext* browser_context,
+      StoragePartitionImpl* storage_partition_impl,
+      bool is_for_guests_only);
+
   ~RenderProcessHostImpl() override;
 
   // RenderProcessHost implementation (public portion).
@@ -361,6 +363,11 @@ class CONTENT_EXPORT RenderProcessHostImpl
   class ConnectionFilterController;
   class ConnectionFilterImpl;
 
+  // Use CreateRenderProcessHostImpl() instead.
+  RenderProcessHostImpl(BrowserContext* browser_context,
+                        StoragePartitionImpl* storage_partition_impl,
+                        bool is_for_guests_only);
+
   // Initializes a new IPC::ChannelProxy in |channel_|, which will be connected
   // to the next child process launched for this host, if any.
   void InitializeChannelProxy();