PNaCl: Add --force-pnacl-subzero to chrome://flags .

Publicizing this already-existing flag makes it much easier to test Subzero, both internally and externally, and especially on an ARM-based Chromebook.

BUG=598970

Review URL: https://codereview.chromium.org/1838083005

Cr-Commit-Position: refs/heads/master@{#384297}
3 files changed