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Non-fast scrollable regions are regions on a cc::Layer where the
compositor should not handle scroll events. These are primarily for non-
composited scrollers but are also used for resize controls and plugins.
This patch paints non-fast scrollable regions for plugins that have
blocking wheel event handlers. A design doc for this approach of
painting non-fast scrollable regions is at:
https://docs.google.com/document/d/1IyYJ6bVF7KZq96b_s5NrAzGtVoBXn_LQnya9y4yT3iw/view#heading=h.rrlzkp4v3huj

This patch shares the pattern used by non-composited scrollers of
emitting ScrollHitTestDisplayItems. ScrollHitTestDisplayItem has a field
for supporting both scrolling (kScrollHitTest) and non-scrolling
(kPluginScrollHitTest) usecases. This field was required because it will
be possible to have two ScrollHitTestDisplayItems for one scroller: one
for non-composited scrolling, and a second for a resizer.

ScrollingCoordinatorTest::NonFastScrollableRegionsForPlugins is the
interesting new test. It shows that PaintNonFastScrollableRegions
matches the current behavior for non-fixed plugins, and shows that fixed
plugin non-fast regions are now painted in a different layer. Painting
the fixed non-fast region into the fixed layer fixes a bug where the
non-fast regions would not be updated after compositor changes such as
scrolling.

Bug: 864567
Change-Id: If72e084241d1684e54d4c02e5d1efc04d91cc858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729551Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683165}