Async DNS: Remove toggle from about:flags

Async DNS is fairly stable at the moment, so we don't really need the
toggle in about:flags anymore. (Note that the --enable-async-dns and
--disable-async-dns command-line flags will still work for now.)

BUG=432236

Review URL: https://codereview.chromium.org/840933003

Cr-Commit-Position: refs/heads/master@{#310552}

build/ios/grit_whitelist.txt

@@ -505,8 +505,6 @@ 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

@@ -6013,12 +6013,6 @@ 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

@@ -667,14 +667,6 @@ 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,