[pinch-to-zoom] Allow pinch viewport size to be different from main frame.

This patch allows the pinch viewport's unscaled size to be set. In normal
operation, the pinch viewport, when unscaled, will be the same size as
the main frame. However, for features like the virtual keyboard, we want
the ability to shrink the pinch viewport so that content under the keyboard
can be scrolled into view.

BUG=349941

Review URL: https://codereview.chromium.org/239353005

git-svn-id: svn://svn.chromium.org/blink/trunk@172083 bbb929c8-8fbe-4397-9dbb-9b2b20218538