Set PinchViewport's scroll layer on FrameView attachment.

https://codereview.chromium.org/591313007/ removed the call from setView
to PinchViewport::mainFrameDidChangeSize as it would clobber the saved
scroll state in the current history item. This turned out to be needed
as sometimes the scroll layer will not get resized on navigation, allowing
the pinch viewport to be scrolled outside the FrameView's bounds. This CL
adds a special attachment call to PinchViewport that sets the size on the
scroll layer without the history clobbering side-effect.

BUG=423189

Review URL: https://codereview.chromium.org/663083006

git-svn-id: svn://svn.chromium.org/blink/trunk@184135 bbb929c8-8fbe-4397-9dbb-9b2b20218538