Enable setting ViewportMeta and ResizesAreOrientationChange from flags.

These settings were added in https://codereview.chromium.org/40423003
(Blink) and https://codereview.chromium.org/38793007 (Chromium) but the
Blink side hardcoded default values until the Chromium side landed. This
CL now hooks up the Chromium flags to the Blink settings.

BUG=232102

Review URL: https://codereview.chromium.org/74943009

git-svn-id: svn://svn.chromium.org/blink/trunk@163201 bbb929c8-8fbe-4397-9dbb-9b2b20218538