setViewportEnabled on InternalSettings

Added possibility to --enable-viewport from internals.settings in LayoutTests.
Currently viewport can only be tested through configurationForViewport which
makes it impossible to test certain codepaths and more complicated cases. This
CL is a pre-condition for a test in another CL I am preparing.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22747002

git-svn-id: svn://svn.chromium.org/blink/trunk@156301 bbb929c8-8fbe-4397-9dbb-9b2b20218538