[Prerender] Restore request priorities when swapped in

chrome/browser/prerender/prerender_resource_throttle.cc

Index: chrome/browser/prerender/prerender_resource_throttle.cc
diff --git a/chrome/browser/prerender/prerender_resource_throttle.cc b/chrome/browser/prerender/prerender_resource_throttle.cc
index afd908fe6db2c2b387927c3fea5619109a906278..b6ed41fdabf0878b3bce20130d8cb5f42a0cec50 100644
--- a/chrome/browser/prerender/prerender_resource_throttle.cc
+++ b/chrome/browser/prerender/prerender_resource_throttle.cc
@@ -12,6 +12,7 @@
 #include "chrome/browser/prerender/prerender_manager.h"
 #include "chrome/browser/prerender/prerender_util.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/browser/resource_dispatcher_host.h"
 #include "content/public/browser/web_contents.h"
 #include "net/base/load_flags.h"
 #include "net/http/http_response_headers.h"
@@ -85,7 +86,29 @@ void PrerenderResourceThrottle::OverridePrerenderContentsForTesting(
 PrerenderResourceThrottle::PrerenderResourceThrottle(net::URLRequest* request)
     : request_(request),
       load_flags_(net::LOAD_NORMAL),
-      prerender_throttle_info_(new PrerenderThrottleInfo()) {}
+      prerender_throttle_info_(new PrerenderThrottleInfo()) {
+// Priorities for prerendering requests are lowered, to avoid competing with
+// other page loads, except on Android where this is less likely to be a
+// problem. In some cases, this may negatively impact the performance of
+// prerendering, see https://crbug.com/652746 for details.
+#if !defined(OS_ANDROID)
+  // Requests with the IGNORE_LIMITS flag set (i.e., sync XHRs)
+  // should remain at MAXIMUM_PRIORITY.
+  if (request_->load_flags() & net::LOAD_IGNORE_LIMITS) {
+    DCHECK_EQ(request_->priority(), net::MAXIMUM_PRIORITY);
+  } else if (request_->priority() != net::IDLE) {
+    original_request_priority_ = request_->priority();
+    // In practice, the resource scheduler does not know about the request yet,
+    // and it falls back to calling request_->SetPriority(), so it would be
+    // possible to do just that here. It is cleaner and more robust to go
+    // through the resource dispatcher host though.
+    if (content::ResourceDispatcherHost::Get()) {
+      content::ResourceDispatcherHost::Get()->ReprioritizeRequest(request_,
+                                                                  net::IDLE);
+    }
+  }
+#endif  // OS_ANDROID
+}
 
 PrerenderResourceThrottle::~PrerenderResourceThrottle() {}
 
@@ -94,6 +117,7 @@ void PrerenderResourceThrottle::WillStartRequest(bool* defer) {
   const content::ResourceRequestInfo* info =
       content::ResourceRequestInfo::ForRequest(request_);
   *defer = true;
+
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&PrerenderResourceThrottle::WillStartRequestOnUI, AsWeakPtr(),
@@ -149,6 +173,16 @@ void PrerenderResourceThrottle::ResumeHandler() {
   Resume();
 }
 
+void PrerenderResourceThrottle::ResetResourcePriority() {
+  if (!original_request_priority_)
+    return;
+
+  if (content::ResourceDispatcherHost::Get()) {
+    content::ResourceDispatcherHost::Get()->ReprioritizeRequest(
+        request_, original_request_priority_.value());
+  }
+}
+
 // static
 void PrerenderResourceThrottle::WillStartRequestOnUI(
     const base::WeakPtr<PrerenderResourceThrottle>& throttle,
@@ -202,9 +236,16 @@ void PrerenderResourceThrottle::WillStartRequestOnUI(
       // Delay icon fetching until the contents are getting swapped in
       // to conserve network usage in mobile devices.
       prerender_contents->AddResourceThrottle(throttle);
+
+      // No need to call AddIdleResource() on Android.
       return;
 #endif
     }
+
+#if !defined(OS_ANDROID)
+    if (!cancel)
+      prerender_contents->AddIdleResource(throttle);
+#endif
   }
 
   BrowserThread::PostTask(