Prepare //build/module_args/mojo.gni for upcoming Mojo rolls

These variables will start being used by BUILD.gn files in the Mojo
repo shortly, so ensure that Chromium has the right values set for them.

Review URL: https://codereview.chromium.org/921763002

Cr-Commit-Position: refs/heads/master@{#315991}
1 file changed