Revert CL 117933003. Re-add resource speculative prefetching code.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index d6dc7cad240d4c8f4f0b354944f5f3553b02bd86..a04c81a0387450f8d957b7dc64dd0248b694ebe7 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -1080,6 +1080,20 @@ const char kSimulateOutdated[]               = "simulate-outdated";
 // Simulates that current version is outdated and auto-update is off.
 const char kSimulateOutdatedNoAU[]           = "simulate-outdated-no-au";
 
+// Speculative resource prefetching.
+const char kSpeculativeResourcePrefetching[] =
+    "speculative-resource-prefetching";
+
+// Speculative resource prefetching is disabled.
+const char kSpeculativeResourcePrefetchingDisabled[] = "disabled";
+
+// Speculative resource prefetching will only learn about resources that need to
+// be prefetched but will not prefetch them.
+const char kSpeculativeResourcePrefetchingLearning[] = "learning";
+
+// Speculative resource prefetching is enabled.
+const char kSpeculativeResourcePrefetchingEnabled[] = "enabled";
+
 // Specifies the URL where spelling service feedback data will be sent instead
 // of the default URL. This switch is for temporary testing only.
 // TODO(rouslan): Remove this flag when feedback testing is complete. Revisit by