[DO NOT SUBMIT] Testing a reverted CL to see if changes catch regression.

[DO NOT SUBMIT] Testing a reverted CL to see if changes catch regression.

crbug.com/665220

MainFrame scrollbars should work with RFV instead of FV

This CL does the following things:
- FV::updateScrollbarGeometry and part of FV::computeScrollbarExistence is moved into
scrollbar manager.
- For MainFrame, the scroller that the scrollbars works with is changed to be RFV from FV
- To accommodate for this, a bunch of methods in Scrollbar that used to call into FV now
call into RFV, which in turn calls the corresponding methods on its layout viewport. An
exception to this is Tickmarks. Tickmarks for MainFrame are now stored in RFV.
- The isOverlayScrollbar method was needlessly implemented in FV. This CL removes this.

BUG=456861
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Committed: https://crrev.com/322db3d6f1f9eb4b9cd6a0054999718c24b1f076
Cr-Commit-Position: refs/heads/master@{#431683}

patch from issue 2454913003 at patchset 350001 (http://crrev.com/2454913003#ps350001)