Add a command line option to the running of some file manager tests

These file manager tests dispatch untrusted events at the DOM. Blink
is changing the default for this option.

Ultimately these tests need to be reworked so they don't dispatch
default actions.

BUG=520519

Review-Url: https://codereview.chromium.org/2088003002
Cr-Commit-Position: refs/heads/master@{#401141}
1 file changed