Make Xvfb for browser_tests dependent on use_x11 instead of OS=="linux"

chrome/browser_tests.isolate

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'conditions': [
-    ['OS=="linux"', {
+    ['use_x11==1', {
       'variables': {
         'command': [
           '../testing/xvfb.py',
           '<(PRODUCT_DIR)',
           '<(PRODUCT_DIR)/browser_tests<(EXECUTABLE_SUFFIX)',
           '--test-launcher-bot-mode',
           '--asan=<(asan)',
           '--lsan=<(lsan)',
           '--msan=<(msan)',
           '--tsan=<(tsan)',
@@ skipping 220 matching lines @@
           'browser/chromeos/login/test/https_forwarder.py',
         ],
       },
     }],
   ],
   'includes': [
     '../base/base.isolate',
     'chrome.isolate',
   ],
 }