Revert "Async DNS: Remove toggle from about:flags"

This reverts commit eabf1f5b.

Removing the toggle in about:flags breaks a workaround for users who
have nonstandard nsswitch configurations for hosts.

Revert the removal until we properly check nsswitch.conf before
assuming a standard configuration.

BUG=117655

Review URL: https://codereview.chromium.org/987133003

Cr-Commit-Position: refs/heads/master@{#319725}

build/ios/grit_whitelist.txt

@@ -514,6 +514,8 @@ IDS_FLAGS_DISABLE_WEBRTC_NAME
 IDS_FLAGS_ENABLE
 IDS_FLAGS_ENABLE_APPS_SHOW_ON_FIRST_PAINT_DESCRIPTION
 IDS_FLAGS_ENABLE_APPS_SHOW_ON_FIRST_PAINT_NAME
+IDS_FLAGS_ENABLE_ASYNC_DNS_DESCRIPTION
+IDS_FLAGS_ENABLE_ASYNC_DNS_NAME
 IDS_FLAGS_ENABLE_CARRIER_SWITCHING
 IDS_FLAGS_ENABLE_CARRIER_SWITCHING_DESCRIPTION
 IDS_FLAGS_ENABLE_CONTEXTUAL_SEARCH

chrome/app/generated_resources.grd

@@ -6121,6 +6121,12 @@ Keep your key file in a safe place. You will need it to create new versions of y
       <message name="IDS_FLAGS_ENABLE_SPDY4_DESCRIPTION" desc="Description for the flag to enable SPDY/4.">
         Enable SPDY/4, which is the HTTP/2 standard. Currently experimental.
       </message>
+      <message name="IDS_FLAGS_ENABLE_ASYNC_DNS_NAME" desc="Title for the flag to enable asynchronous DNS client.">
+        Built-in Asynchronous DNS
+      </message>
+      <message name="IDS_FLAGS_ENABLE_ASYNC_DNS_DESCRIPTION" desc="Description for the flag to enable asynchronous DNS client.">
+        Enable experimental asynchronous DNS client.
+      </message>
       <message name="IDS_FLAGS_DISABLE_MEDIA_SOURCE_NAME" desc="Title for the flag to disable the Media Source API.">
         Disable Media Source API.
       </message>

chrome/browser/about_flags.cc

@@ -677,6 +677,14 @@ const Experiment kExperiments[] = {
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kEnableSpdy4)
   },
+  {
+    "enable-async-dns",
+    IDS_FLAGS_ENABLE_ASYNC_DNS_NAME,
+    IDS_FLAGS_ENABLE_ASYNC_DNS_DESCRIPTION,
+    kOsWin | kOsMac | kOsLinux | kOsCrOS,
+    ENABLE_DISABLE_VALUE_TYPE(switches::kEnableAsyncDns,
+                              switches::kDisableAsyncDns)
+  },
   {
     "disable-media-source",
     IDS_FLAGS_DISABLE_MEDIA_SOURCE_NAME,