Enable the OOP PDF viewer as the default

This relands https://codereview.chromium.org/748553003/ which got incorrectly
merged by the commit bot :(

TBR=sammc@chromium.org
BUG=303491

Review URL: https://codereview.chromium.org/850633002

Cr-Commit-Position: refs/heads/master@{#311225}
1 file changed