This sets the default for the feature, as chromium expects
it: It is turned on for all platforms except ios.

Chromium's build_override can be removed after this.

This will also allow to override the value as a gn arg.

BUG=chromium:474921,chromium:616034
NOTRY=true

Review-Url: https://codereview.chromium.org/2025803003
Cr-Commit-Position: refs/heads/master@{#36666}