Revert "Separate the scrolling code out of RenderLayer"

This reverts commit ebb99ce602365bba2c70e3791e567ad3da8f77b3 (r154761).

Breaks two webkit_unit_tests:
CrossPlatformAccessibilityBrowserTest.SelectedEditableTextAccessibility and
CrossPlatformAccessibilityBrowserTest.UnselectedEditableTextAccessibility

TBR=jchaffraix@chromium.org
BUG=260899

Review URL: https://codereview.chromium.org/20051002

git-svn-id: svn://svn.chromium.org/blink/trunk@154769 bbb929c8-8fbe-4397-9dbb-9b2b20218538