Enable spare RenderProcessHost to be preinitialized.

content/public/browser/render_process_host.h

Index: content/public/browser/render_process_host.h
diff --git a/content/public/browser/render_process_host.h b/content/public/browser/render_process_host.h
index 778f93da8ceffbb9c6fa48d053c098f332289d64..b8a7b55f61bd421bd9a08e65eef0d330cb55e721 100644
--- a/content/public/browser/render_process_host.h
+++ b/content/public/browser/render_process_host.h
@@ -377,6 +377,12 @@ class CONTENT_EXPORT RenderProcessHost : public IPC::Sender,
 
   // Static management functions -----------------------------------------------
 
+  // Possibly start an unbound, spare RenderProcessHost. A subsequent call to
+  // CreateRenderProcessHost with a matching browser_context may use this
+  // preinitialized RenderProcessHost, improving performance.

[Charlie Reis, 2017/06/25 23:48:28]

Please elaborate (as mentioned in the comment in render_process_host_impl.cc). 
There should be a fair amount here describing the restrictions, how it's meant
to be used, etc.

[mattcary, 2017/06/26 14:45:07]

Done.

+  static void WarmupSpareRenderProcessHost(
+      content::BrowserContext* browser_context);
+
   // Flag to run the renderer in process.  This is primarily
   // for debugging purposes.  When running "in process", the
   // browser maintains a single RenderProcessHost which communicates