Fix bug in shut down of HTMLViewer

mojo/tools/data/apptests

 # This file contains a list of Mojo gtest unit tests.
 #
 # This must be valid Python. It may use the |config| global that will be a
 # mopy.config.Config object, and must set a |tests| global that will contain the
 # list of tests to run.
 #
 # The entries in |tests| are dictionaries of the following form:
 #   {
 #     # Required URL for apptest.
 #     'test': 'mojo:test_app_url',
@@ skipping 38 matching lines @@
   view_manager_apptests,
 ]
 
 # TODO(msw): Get these tests passing on Android too. http://crbug.com/486220
 if config.target_os != config.OS_ANDROID:
   tests += [
     {
       'test': 'mojo:filesystem_apptests',
       'type': 'gtest_isolated',
     },
-# TODO(sky): reenable once 531120 is fixed.
-#    {
-#      'test': 'mojo:html_viewer_apptests',
-#      'name': 'mojo:html_viewer_apptests',
-#      'type': 'gtest_isolated',
-#      'args': ['--enable-html-viewer-test-interface',
-#               '--use-x11-test-config',
-#               '--override-use-gl-with-osmesa-for-tests']
-#    },
-#    {
-#      'test': 'mojo:html_viewer_apptests',
-#      'name': 'mojo:html_viewer_apptests new frame',
-#      'type': 'gtest_isolated',
-#      'args': ['--oopif-always-create-new-frame-tree',
-#               '--enable-html-viewer-test-interface',
-#               '--use-x11-test-config',
-#               '--override-use-gl-with-osmesa-for-tests']
-#    },
+    {
+      'test': 'mojo:html_viewer_apptests',
+      'name': 'mojo:html_viewer_apptests',
+      'type': 'gtest_isolated',
+      'args': ['--enable-html-viewer-test-interface',
+               '--use-x11-test-config',
+               '--override-use-gl-with-osmesa-for-tests']
+    },
+    {
+      'test': 'mojo:html_viewer_apptests',
+      'name': 'mojo:html_viewer_apptests new frame',
+      'type': 'gtest_isolated',
+      'args': ['--oopif-always-create-new-frame-tree',
+               '--enable-html-viewer-test-interface',
+               '--use-x11-test-config',
+               '--override-use-gl-with-osmesa-for-tests']
+    },
     # TODO(ben): These tests are disabled pending mandoline:browser refactoring.
     #{
     #  'test': 'mojo:mandoline_browser_apptests',
     #  'type': 'gtest_isolated',
     #  'args': ['--use-headless-config']
     #},
     # TODO(xhwang): Fix and enable mojo:media_pipeline_integration_apptests.
     # http://crbug.com/501417
     {
       'test': 'mojo:media_apptests',
       'type': 'gtest_isolated',
     },
     {
       'test': 'mojo:sql_apptests',
       'type': 'gtest_isolated',
     },
     {
       'test': 'mojo:web_view_apptests',
       'type': 'gtest_isolated',
       'args': ['--use-x11-test-config',
                '--override-use-gl-with-osmesa-for-tests']
     },
   ]