Made PinchViewport a ScrollableArea.

Implemented ScrollableArea on PinchViewport and hooked it up
to all the layer scrolling mechanisms. This removes the need
for DistributeScrollOffsetToViewports on the layer tree.

Also did a little cleanup with member names and namespacing.

This is a 2-sided patch, the other CL:
https://codereview.chromium.org/210543002

BUG=349941

Review URL: https://codereview.chromium.org/199253002

git-svn-id: svn://svn.chromium.org/blink/trunk@170110 bbb929c8-8fbe-4397-9dbb-9b2b20218538