Renamed PinchViewports to PinchViewport.

It's really only responsible for the inner viewport and will be more
so as it's moved deeper into Blink.

BUG=349941

Review URL: https://codereview.chromium.org/184793004

git-svn-id: svn://svn.chromium.org/blink/trunk@168664 bbb929c8-8fbe-4397-9dbb-9b2b20218538