predictors: Add a origin for prefetch requests, remove the URL/host settings.

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 4aaa9f02bf9a59715cae07f22e1c44c0ed97c2e8..ec81d9a31c518b9b0ea60ce734f899b4b28865f3 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -893,6 +893,10 @@ const char kSpeculativeResourcePrefetchingDisabled[] = "disabled";
 // Speculative resource prefetching is enabled.
 const char kSpeculativeResourcePrefetchingEnabled[] = "enabled";
 
+// Speculative resource prefetching is enabled for external requests.
+const char kSpeculativeResourcePrefetchingEnabledExternal[] =
+    "enabled-external-only";
+
 // Speculative resource prefetching will only learn about resources that need to
 // be prefetched but will not prefetch them.
 const char kSpeculativeResourcePrefetchingLearning[] = "learning";