Support subpixel layout for borders.

third_party/WebKit/Source/core/css/CSSProperties.in

 // This file specifies all the CSS properties we support and the necessary
 // information for our code generation. The various supported arguments
 // are described below with example usage
 
 
 // - alias_for=other-property
 // Properties specifying alias_for should be virtually identical to the
 // properties they alias. Minor parsing differences are allowed as long as
 // the CSSValues created are of the same format of the aliased property.
 
@@ skipping 193 matching lines @@
 background-position-x interpolable, custom_all
 background-position-y interpolable, custom_all
 background-repeat-x custom_all
 background-repeat-y custom_all
 background-size interpolable, custom_all
 baseline-shift interpolable, svg, custom_inherit, custom_value
 border-bottom-color interpolable, custom_all
 border-bottom-left-radius interpolable, initial=initialBorderRadius, converter=convertRadius, api_class=CSSPropertyAPIBorderRadius
 border-bottom-right-radius interpolable, initial=initialBorderRadius, converter=convertRadius, api_class=CSSPropertyAPIBorderRadius
 border-bottom-style type_name=EBorderStyle, initial=initialBorderStyle
-border-bottom-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<unsigned>
+border-bottom-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<float>

[pdr., 2017/01/24 06:57:31]

Should outline-width be changed in a similar way (as a followup)?

[Karl Øygard, 2017/01/24 22:42:58]

Yes, that would make sense.  I debated including outline-width and
column-rule-width in this patch, since their underlying data structures
now use floats, but opted against it since the changes are already quite
large.

 border-collapse inherited, independent, keyword_only, keywords=[separate|collapse], initial_keyword=separate

[pdr., 2017/01/24 06:57:31]

Some of the SVG test results seem like regressions (e.g.,
third_party/WebKit/LayoutTests/platform/mac/svg/zoom/page/zoom-background-images-expected.png).

Similarly, the zoom test results seem suspect to me (e.g.,
third_party/WebKit/LayoutTests/platform/win/fast/forms/color/color-suggestion-picker-appearance-zoom125-expected.png).
Shouldn't these get strictly better?

[Karl Øygard, 2017/01/24 22:42:58]

I'll recheck and verify that the new results are as expected.

The SVG tests in particular are a little tricky, since their views have int
sizes, and don't always fit well in with subpixel layout.  That was an
issue before this patch as well, but subpixel borders may in some cases
make it more obvious.

[Karl Øygard, 2017/01/27 20:19:27]

Yes, something's amiss here, investigating.
They look mostly fine to me, what do you see that you don't like?  The form
fields just shift around a bit, I assumed due to borders that now become
larger.  I'll admit I didn't investigate closely--I'm not sure how and where
these pickers are implemented.

[Karl Øygard, 2017/01/28 16:46:27]

I discovered one issue with repeating background images and sub-pixel
rendering.  This issue was already present, but was much less likely to be
triggered before.  I've created a separate bug report for that  in
https://bugs.chromium.org/p/chromium/issues/detail?id=686435

 border-image-outset interpolable, custom_all
 border-image-repeat custom_all
 border-image-slice interpolable, custom_all
 border-image-source interpolable, custom_value, typedom_types=[Image], keywords=[none]
 border-image-width interpolable, custom_all
 border-left-color interpolable, custom_all
 border-left-style type_name=EBorderStyle, initial=initialBorderStyle
-border-left-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<unsigned>
+border-left-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<float>
 border-right-color interpolable, custom_all
 border-right-style type_name=EBorderStyle, initial=initialBorderStyle
-border-right-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<unsigned>
+border-right-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<float>
 border-top-color interpolable, custom_all
 border-top-left-radius interpolable, initial=initialBorderRadius, converter=convertRadius, api_class=CSSPropertyAPIBorderRadius
 border-top-right-radius interpolable, initial=initialBorderRadius, converter=convertRadius, api_class=CSSPropertyAPIBorderRadius
 border-top-style type_name=EBorderStyle, initial=initialBorderStyle
-border-top-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<unsigned>, typedom_types=[Length], keywords=[thin|medium|thick], supports_percentage
+border-top-width interpolable, initial=initialBorderWidth, converter=convertLineWidth<float>, typedom_types=[Length], keywords=[thin|medium|thick], supports_percentage
 bottom typedom_types=[Length], keywords=[auto], supports_percentage, interpolable, initial=initialOffset, converter=convertLengthOrAuto
 box-shadow interpolable, converter=convertShadowList
 box-sizing
 break-after type_name=EBreak
 break-before type_name=EBreak
 break-inside type_name=EBreak
 buffered-rendering svg
 caption-side inherited, keyword_only, independent, keywords=[top|bottom|left|right], initial_keyword=top
 caret-color interpolable, inherited, custom_all, api_class
 clear
@@ skipping 413 matching lines @@
 -webkit-text-size-adjust alias_for=text-size-adjust
 -webkit-transform alias_for=transform
 -webkit-transform-origin alias_for=transform-origin
 -webkit-transform-style alias_for=transform-style
 -webkit-transition alias_for=transition
 -webkit-transition-delay alias_for=transition-delay
 -webkit-transition-duration alias_for=transition-duration
 -webkit-transition-property alias_for=transition-property
 -webkit-transition-timing-function alias_for=transition-timing-function
 -webkit-user-select alias_for=user-select