[Pinch-to-zoom] Moved scale factor into PinchViewport

Page scale now owned and synchronized in PinchViewport class
when using --enable-pinch-virtual-viewport.

BUG=349941

Review URL: https://codereview.chromium.org/225303014

git-svn-id: svn://svn.chromium.org/blink/trunk@171596 bbb929c8-8fbe-4397-9dbb-9b2b20218538