Turns on use_mojo=1 for linux.

content/content_unittests.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': [
     ['OS=="android" or OS=="linux" or OS=="mac" or OS=="win"', {
       'variables': {
         'isolate_dependency_untracked': [
           '../media/test/data/',
           '../webkit/data/',
@@ skipping 28 matching lines @@
           '--brave-new-test-launcher',
           '--test-launcher-bot-mode',
         ],
         'isolate_dependency_tracked': [
           '../testing/xvfb.py',
           '<(PRODUCT_DIR)/libffmpegsumo.so',
           '<(PRODUCT_DIR)/xdisplaycheck<(EXECUTABLE_SUFFIX)',
         ],
       },
     }],
+    ['OS=="linux" and use_mojo==1', {
+      'variables': {
+        'isolate_dependency_tracked': [
+          '<(PRODUCT_DIR)/lib/libmojo_system.so',
+        ],
+      },
+    }],
     ['OS=="mac"', {
       'variables': {
         'command': [
           '../testing/test_env.py',
           '<(PRODUCT_DIR)/content_unittests<(EXECUTABLE_SUFFIX)',
           '--brave-new-test-launcher',
           '--test-launcher-bot-mode',
         ],
         'isolate_dependency_tracked': [
           '<(PRODUCT_DIR)/ffmpegsumo.so',
@@ skipping 12 matching lines @@
           '<(PRODUCT_DIR)/ffmpegsumo.dll',
           '<(PRODUCT_DIR)/ui_test.pak',
         ],
       },
     }],
   ],
   'includes': [
     '../third_party/icu/icu.isolate',
   ],
 }