Add ability to create solid color scrollbars in ScrollingCoordinator.

This CL adds the ability to create solid color scrollbars within ScrollingCoordinator,
based on the current value of the settings useOverlayScrollbars and useSolidColorScrollbars.
This functionality is made available in the class interface to facilitate
access from WebViewImpl in subsequent pinch-zoom CLs.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23620023

git-svn-id: svn://svn.chromium.org/blink/trunk@157254 bbb929c8-8fbe-4397-9dbb-9b2b20218538