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}
1 file changed