add missing content_test_mojo_bindings gyp dep in content_tests.gypi

This should avoid possible build flakiness.

Found using this proposed ninja patch:
https://github.com/martine/ninja/pull/1031

BUG=536641

Review URL: https://codereview.chromium.org/1371073002

Cr-Commit-Position: refs/heads/master@{#351063}

content/content_shell.gypi

@@ -39,6 +39,7 @@
         'content.gyp:content_resources',
         'content.gyp:content_utility',
         'content_shell_resources',
+        'content_test_mojo_bindings',
         'copy_test_netscape_plugin',
         'layouttest_support_content',
         '../base/base.gyp:base',