Disable FTA when the maximum multiplier is 1 (e.g., pages with meta viewport width=device-width).

Also handle zero frame width correctly (happens occasionally per crbug.com/340987), and simplify the LayoutScope interface.

BUG=347384
NOTRY=true

Review URL: https://codereview.chromium.org/180743004

git-svn-id: svn://svn.chromium.org/blink/trunk@168397 bbb929c8-8fbe-4397-9dbb-9b2b20218538