Register data_reduction_proxy prefs on iOS.

ios/chrome/browser/prefs/browser_prefs.mm

Index: ios/chrome/browser/prefs/browser_prefs.mm
diff --git a/ios/chrome/browser/prefs/browser_prefs.mm b/ios/chrome/browser/prefs/browser_prefs.mm
index ec6d7a90d63a0c4d4c45a349209d36d1308ec72b..146e362303415e152943b3406ed5eaf2a759ef77 100644
--- a/ios/chrome/browser/prefs/browser_prefs.mm
+++ b/ios/chrome/browser/prefs/browser_prefs.mm
@@ -7,6 +7,8 @@
 #include "base/prefs/pref_service.h"
 #include "components/autofill/core/browser/autofill_manager.h"
 #include "components/content_settings/core/browser/host_content_settings_map.h"
+#include "components/data_reduction_proxy/core/browser/data_reduction_proxy_prefs.h"
+#include "components/data_reduction_proxy/core/common/data_reduction_proxy_pref_names.h"
 #include "components/dom_distiller/core/distilled_page_prefs.h"
 #include "components/enhanced_bookmarks/bookmark_server_cluster_service.h"
 #include "components/gcm_driver/gcm_channel_status_syncer.h"
@@ -74,6 +76,13 @@ void RegisterLocalStatePrefs(PrefRegistrySimple* registry) {
   [OmniboxGeolocationLocalState registerLocalState:registry];
   [MemoryDebuggerManager registerLocalState:registry];
 
+  // TODO(crbug.com/557814): data_reduction_proxy::prefs::kDataReductionProxy
+  // should be registered by data_reduction_proxy::RegisterPrefs() instead of
+  // here.
+  registry->RegisterStringPref(data_reduction_proxy::prefs::kDataReductionProxy,
+                               std::string());
+  data_reduction_proxy::RegisterPrefs(registry);
+
   // TODO(shreyasv): Remove this in M49 as almost all users would have the
   // "do not backup" bit set by then. crbug.com/489865.
   registry->RegisterBooleanPref(prefs::kOTRStashStatePathSystemBackupExcluded,