Enable HttpFormWarning feature for browser tests

The HttpFormWarning is enabled by fieldtrial_testing_config.json but
that configuration is not used by the official builders, causing the
ShowFormNotSecureOnUsernameField browser test to fail when run on the
builders.

Explicitly enable the feature for the affected browser tests.

BUG=693717

Review-Url: https://codereview.chromium.org/2709703003
Cr-Commit-Position: refs/heads/master@{#452026}
1 file changed