Upon further examination, I see this is only true when root layer scrolling is enabled.

Scratch that last comment; it's true with or without RLS.

To clarify:

GeometryMapper always applies the overflow clip of the ancestor.  Without root layer scrolling, frame-level overflow is handled in a custom way, so the frame-level overflow clip is not applied when mapping up to the LayoutView.  With root layer scrolling enabled, frame-level overflow is handled the same as the overflow on any other LayoutBox, so frame-level overflow clip *is* applied when mapping up to the LayoutView.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/abb4cf14dd75848473eaf8706fbca3be284f027a

commit abb4cf14dd75848473eaf8706fbca3be284f027a
Author: chrishtr <chrishtr@chromium.org>
Date: Thu Sep 01 22:06:42 2016

Define contentsProperties on ObjectPaintProperties, for use in paint invalidation.

BUG= 638415 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2292273003
Cr-Commit-Position: refs/heads/master@{#416079}

[modify] https://crrev.com/abb4cf14dd75848473eaf8706fbca3be284f027a/third_party/WebKit/Source/core/core.gypi
[add] https://crrev.com/abb4cf14dd75848473eaf8706fbca3be284f027a/third_party/WebKit/Source/core/paint/ObjectPaintProperties.cpp
[modify] https://crrev.com/abb4cf14dd75848473eaf8706fbca3be284f027a/third_party/WebKit/Source/core/paint/ObjectPaintProperties.h
[modify] https://crrev.com/abb4cf14dd75848473eaf8706fbca3be284f027a/third_party/WebKit/Source/core/paint/PaintInvalidator.cpp
[modify] https://crrev.com/abb4cf14dd75848473eaf8706fbca3be284f027a/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp