[RootLayerScrolls] Cleanup Frame/Contents conversions.

Conversion between 'frame' and 'contents' coordinate spaces is now a
no-op. This CL settles on 'frame' as the term d'art for the coordinate
space of the frame, so ViewportToContents becomes ViewportToFrame, and
vice versa.

Also, this adds a couple of missing float overrides for still-useful
conversion methods. Arbitrarily converting between LayoutUnit and
float can be a source of bugs.


Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Id687f88988e6e27ea75027026377d9464bee3406
Reviewed-on: https://chromium-review.googlesource.com/1099996
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567214}
22 files changed