Add LOAD_PREFETCH flag for all requests sent by resource prefetcher

1. This allows other constraints about prefetched resources to be applied.
2. Together with another CL https://codereview.chromium.org/793823002/, the prefetched resources can temporarily bypass revalidation.

BUG=405690, 406209

Review URL: https://codereview.chromium.org/891343007

Cr-Commit-Position: refs/heads/master@{#315369}

chrome/browser/predictors/resource_prefetcher.cc

@@ -10,6 +10,7 @@
 #include "base/stl_util.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/base/io_buffer.h"
+#include "net/base/load_flags.h"
 #include "net/base/request_priority.h"
 #include "net/url_request/url_request_context.h"
 
@@ -134,6 +135,7 @@ void ResourcePrefetcher::SendRequest(Request* request) {
   url_request->set_method("GET");
   url_request->set_first_party_for_cookies(navigation_id_.main_frame_url);
   url_request->SetReferrer(navigation_id_.main_frame_url.spec());
+  url_request->SetLoadFlags(url_request->load_flags() | net::LOAD_PREFETCH);
   StartURLRequest(url_request);
 }
 

chrome/browser/predictors/resource_prefetcher_unittest.cc

@@ -9,6 +9,7 @@
 #include "chrome/browser/predictors/resource_prefetcher_manager.h"
 #include "chrome/test/base/testing_profile.h"
 #include "content/public/test/test_browser_thread.h"
+#include "net/base/load_flags.h"
 #include "net/url_request/redirect_info.h"
 #include "net/url_request/url_request.h"
 #include "net/url_request/url_request_test_util.h"
@@ -37,6 +38,7 @@ class TestResourcePrefetcher : public ResourcePrefetcher {
   MOCK_METHOD1(StartURLRequest, void(net::URLRequest* request));
 
   void ReadFullResponse(net::URLRequest* request) override {
+    EXPECT_TRUE(request->load_flags() & net::LOAD_PREFETCH);
     FinishRequest(request, Request::PREFETCH_STATUS_FROM_CACHE);
   }