Always use ENABLE_PLUGINS to indicate plugins support.

content/content_browsertests.isolate

 # Copyright (c) 2013 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': [
     ['use_x11==0', {
       'variables': {
         'command': [
           '../testing/test_env.py',
           '<(PRODUCT_DIR)/content_browsertests<(EXECUTABLE_SUFFIX)',
@@ skipping 59 matching lines @@
         ],
       },
     }],
     ['OS=="linux" and enable_plugins==1', {
       'variables': {
         'files': [
           '<(PRODUCT_DIR)/libppapi_tests.so',
         ],
       },
     }],
+    ['enable_plugins==1', {
+      'variables': {
+        'files': [
+          '../ppapi/tests/test_case.html',
+          '../ppapi/tests/test_page.css',
+        ],
+      },
+    }],
     ['OS=="linux" or OS=="mac" or OS=="win"', {
       'variables': {
         'files': [
           '../net/tools/testserver/',
-          '../ppapi/tests/test_case.html',
-          '../ppapi/tests/test_page.css',
           '../testing/test_env.py',
           '../third_party/pyftpdlib/',
           '../third_party/pywebsocket/',
           '../third_party/tlslite/',
           '<(PRODUCT_DIR)/content_browsertests<(EXECUTABLE_SUFFIX)',
         ],
       },
     }],
     ['OS=="mac"', {
       'variables': {
@@ skipping 40 matching lines @@
         ],
       },
     }],
   ],
   'includes': [
     '../base/base.isolate',
     '../gin/v8.isolate',
     '../ui/gl/gl.isolate',
   ],
 }