Move PageScaleConstraintsSet to Source/web.

This is only used from WebViewImpl and will likely never be depended on
by anything in core/.

NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/26204002

git-svn-id: svn://svn.chromium.org/blink/trunk@159009 bbb929c8-8fbe-4397-9dbb-9b2b20218538