Thanks for looking into this!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ecac02278f03bf594b7ce36b0c53583b7a965565

commit ecac02278f03bf594b7ce36b0c53583b7a965565
Author: Xianzhu Wang <wangxianzhu@chromium.org>
Date: Fri Oct 26 23:09:18 2018

[PE] Fix -1px offset of compositing contents in subpixel-positioned iframe

For a composited object whose layout location is at (10.5, 10.5), we
snap the layer location to (11, 11), and keep a subpixel accumulation
(-0.5, -0.5). RoundIntPoint(LayoutUnit(-0.5, -0.5)) is zero to ensure
that the contents of the composited object will paint at correct
location.

Previously in CompositedLayerMapping::UpdateAfterPartResize() we used
FloatSize::RoundedIntSize() which rounded (-0.5, -0.5) to (-1, -1)
(which is based on the roundf() behavior), causing the iframe's document
layer to be placed at unwanted -1px offset.

Now use LayoutSize::RoundedIntSize() to keep consistent pixel snapping
behavior.

Bug:  898941 
Change-Id: I50b27565850061aec9cffffbe77061570683610b
Reviewed-on: https://chromium-review.googlesource.com/c/1302327
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603241}
[add] https://crrev.com/ecac02278f03bf594b7ce36b0c53583b7a965565/third_party/WebKit/LayoutTests/paint/subpixel/subpixel-composited-iframe-expected.html
[add] https://crrev.com/ecac02278f03bf594b7ce36b0c53583b7a965565/third_party/WebKit/LayoutTests/paint/subpixel/subpixel-composited-iframe.html
[modify] https://crrev.com/ecac02278f03bf594b7ce36b0c53583b7a965565/third_party/blink/renderer/core/paint/compositing/composited_layer_mapping.cc