Set enable_hangouts_services_extension in build/common.gypi.

This build flag used to be set from a repository that's going away, so
enabling this flag for official builds has to be moved here.

BUG=482123
R=kjellander@chromium.org,jochen@chromium.org,tommi@chromium.org

Review URL: https://codereview.chromium.org/1147013002

Cr-Commit-Position: refs/heads/master@{#330927}

build/common.gypi

@@ -2394,6 +2394,12 @@
       ['cfi_vptr==1 or cfi_derived_cast==1 or cfi_unrelated_cast==1', {
         'use_lto%': 1,
       }],
+
+      ['branding=="Chrome" and buildtype=="Official"', {
+        'enable_hangout_services_extension%': 1,
+      }, {
+        'enable_hangout_services_extension%': 0,
+      }],
     ],
 
     # The path to the ANGLE library.