Add setting for fitting the content in the viewport width

This will be enabled for Android only, following a patch for
Chromium side, effectively disabling this mobile specific behavior for desktop.

BUG=359047

Review URL: https://codereview.chromium.org/219223003

git-svn-id: svn://svn.chromium.org/blink/trunk@170662 bbb929c8-8fbe-4397-9dbb-9b2b20218538