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This cl implements subpixel layout of borders.  Painting of subpixel
borders now uses rounding to nearest CSS pixel, and is aligned with
Edge.  Handling of device pixel ratios > 1 isn't implemented here,
and is better covered in a separate task.

This does not implement subpixel borders for tables, as that interacts
with table layout width calculations, which would also need to be
adapted to subpixels.  Again, that's better dealt with in a separate
task.

A number of test cases had to be rebaselined, due to minor changes in
layout and/or painting.  Most of these changes appear in tests that uses
zooming, and implicitly have fractional borders.

svg/zoom/page/zoom-replaced-intrinsic-ratio-001.htm now looks a bit
broken, but that's due to an unrelated issue with aspect ratio and
subpixels (in LayoutPart, if I recall correctly).

BUG=227192
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2640143005
Cr-Commit-Position: refs/heads/master@{#449943}