Add enable_mdns flag

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 734e86d0df305373e687dbeedccb7e7316b20d55..5a4802d3492cb0f37d11bc714814169d58cafbeb 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -641,7 +641,7 @@
         }, {
           'test_isolation_mode%': 'noop',
         }],
-        # Whether Android ARM build uses OpenMAX DL FFT.  
+        # Whether Android ARM build uses OpenMAX DL FFT.
         ['OS=="android" and target_arch=="arm" and android_webview_build==0', {
           # Currently only supported on Android ARM, without webview.
           # When enabled, this will also enable WebAudio on Android
@@ -1048,6 +1048,9 @@
     # rlz codes for searches but do not use the library.
     'enable_rlz%': 0,
 
+    # MDNS is disabled by default.
+    'enable_mdns%' : 0,
+
     'conditions': [
       ['OS=="win" and "<!(python <(DEPTH)/build/dir_exists.py <(windows_sdk_default_path))"=="True"', {
         'windows_sdk_path%': '<(windows_sdk_default_path)',
@@ -2108,6 +2111,9 @@
       ['spdy_proxy_auth_property != ""', {
         'defines': ['SPDY_PROXY_AUTH_PROPERTY="<(spdy_proxy_auth_property)"'],
       }],
+      ['enable_mdns==1', {
+        'defines': ['ENABLE_MDNS=1'],
+      }]
     ],  # conditions for 'target_defaults'
     'target_conditions': [
       ['enable_wexit_time_destructors==1', {