Add WebPrescientNetworking to trigger preconnect from Blink.

Source/Platform/chromium/public/WebPrerenderingSupport.h

Index: Source/Platform/chromium/public/WebPrerenderingSupport.h
diff --git a/Source/Platform/chromium/public/WebPrerenderingSupport.h b/Source/Platform/chromium/public/WebPrerenderingSupport.h
index a2672911fa50a83358d2bf4b0996fd5493806ba2..bf80e3ee4c03ab38f889d05c98408f6e4445f637 100644
--- a/Source/Platform/chromium/public/WebPrerenderingSupport.h
+++ b/Source/Platform/chromium/public/WebPrerenderingSupport.h
@@ -32,10 +32,12 @@
 #define WebPrerenderingSupport_h
 
 #include "WebCommon.h"
+#include "WebPreconnectMotivation.h"
 
 namespace WebKit {
 
 class WebPrerender;
+class WebURL;
 
 class WebPrerenderingSupport {
 public:
@@ -43,6 +45,11 @@ public:
     WEBKIT_EXPORT static void shutdown();
     WEBKIT_EXPORT static WebPrerenderingSupport* current();
 
+    // When a page navigation is speculated, preconnect is triggered to hide
+    // session initialization latency to the server providing the page resource.
+    virtual void preconnect(const WebURL& url,
+                            WebPreconnectMotivation motivation) { }

[abarth-chromium, 2013/05/10 16:57:25]

This API shouldn't be coupled with WebPrerenderingSupport.  It should be
directly on Platform, similar to:

 WebKit::Platform::current()->prefetchHostName(hostname);

[kouhei (in TOK), 2013/05/13 01:31:48]

Actually, this patch was first created as an addition to platform API, but I
heard some concerns:
https://codereview.chromium.org/14749005/diff/1/content/renderer/renderer_web...
https://codereview.chromium.org/14746002/#msg5

I think it is best to have prefetching APIs (prefetch DNS, preconnect,
prerendering) to be in same place. I was thinking about having all these in
PrerenderingSupport, then renaming it adequately. 

+
     // A prerender link element is added when it is inserted into a document.
     virtual void add(const WebPrerender&) = 0;