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 9889ce710a4a3ca20fc8997918a0dd460ff3ba0a..7f29ea381375dc8152a995529d1f02a86313e956 100644
--- a/content/browser/renderer_host/render_process_host_impl.h
+++ b/content/browser/renderer_host/render_process_host_impl.h
@@ -114,9 +114,20 @@ 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);
+  // Use the spare RenderProcessHost if it exists, or create a new one. This
+  // should be the usual way to get a new RenderProcessHost.
+  static RenderProcessHost* CreateOrUseSpareRenderProcessHost(
+      BrowserContext* browser_context,
+      StoragePartitionImpl* storage_partition_impl,
+      bool is_for_guests_only);
+
+  // Create a new RenderProcessHost. In most cases
+  // CreateOrUseSpareRenderProcessHost, above, should be used instead.
+  static RenderProcessHost* CreateRenderProcessHost(
+      BrowserContext* browser_context,
+      StoragePartitionImpl* storage_partition_impl,
+      bool is_for_guests_only);
+
   ~RenderProcessHostImpl() override;
 
   // RenderProcessHost implementation (public portion).
@@ -269,6 +280,11 @@ class CONTENT_EXPORT RenderProcessHostImpl
       BrowserContext* browser_context,
       SiteInstanceImpl* site_instance);
 
+  // Cleanup and remove any spare renderer. This should be used when a
+  // navigation has occurred or will be occurring that will not use the spare
+  // renderer and resources should be cleaned up.
+  static void CleanupSpareRenderProcessHost();
+
   static base::MessageLoop* GetInProcessRendererThreadForTesting();
 
   // This forces a renderer that is running "in process" to shut down.
@@ -337,6 +353,18 @@ class CONTENT_EXPORT RenderProcessHostImpl
       RenderProcessHost* render_process_host,
       const GURL& site_url);
 
+  // Return the spare RenderProcessHost, if it exists. There is at most one
+  // globally-used spare RenderProcessHost at any time.
+  static RenderProcessHost* GetSpareRenderProcessHostForTesting();
+
+  // Return true if the host has not been used. This is stronger than IsUnused()
+  // in that it checks if this RPH has ever been used to render at all, rather
+  // than just no being suitable to host a URL that requires a dedicated
+  // process.
+  // TODO(alexmos): can this be unified with IsUnused()? See also
+  // crbug.com/738634.
+  bool HostHasNotBeenUsed();
+
  protected:
   // A proxy for our IPC::Channel that lives on the IO thread.
   std::unique_ptr<IPC::ChannelProxy> channel_;
@@ -363,6 +391,12 @@ class CONTENT_EXPORT RenderProcessHostImpl
   class ConnectionFilterController;
   class ConnectionFilterImpl;
 
+  // Use CreateRenderProcessHost() instead of calling this constructor
+  // directly.
+  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();