Introduce WebRuntimeFeatures::setCSSViewportEnabled() so that it can be called from content::SetRuntimeFeaturesDefaultsAndUpdateFromArgs()
if the "--enable-viewport" command line option is present.

BUG=420535

Review URL: https://codereview.chromium.org/630903002

git-svn-id: svn://svn.chromium.org/blink/trunk@183593 bbb929c8-8fbe-4397-9dbb-9b2b20218538