Enable multiprocess by default on desktop Mandoline.

This also turns on the new EDK by default (we see hangs with the old one) and disables the sandbox while a recent regression is tracked down.

BUG=478251,546644

Committed: https://crrev.com/8627a07c4dcdd84246b419b2f9659be8dc5c4ffd
Cr-Commit-Position: refs/heads/master@{#357360}

Review URL: https://codereview.chromium.org/1406403004

Cr-Commit-Position: refs/heads/master@{#357412}
4 files changed