CSS: computed scroll-snap-type uses shortest serialization

Values like "x proximity" are serialized as "x"
in the computed value.

If no strictness value is specified, proximity is assumed.

Spec: 
https: //drafts.csswg.org/css-scroll-snap-1/#scroll-snap-type
Change-Id: Ic0e0b73b62b17740452562d1aac0a52915db9fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605243Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658466}

third_party/blink/renderer/core/css/properties/computed_style_utils.cc

@@ -2083,6 +2083,8 @@ CSSValue* ComputedStyleUtils::ValueForScrollSnapType(
     const cc::ScrollSnapType& type,
     const ComputedStyle& style) {
   if (!type.is_none) {
+    if (type.strictness == cc::SnapStrictness::kProximity)
+      return CSSIdentifierValue::Create(type.axis);
     return MakeGarbageCollected<CSSValuePair>(
         CSSIdentifierValue::Create(type.axis),
         CSSIdentifierValue::Create(type.strictness),

third_party/blink/web_tests/external/wpt/css/css-scroll-snap/inheritance-expected.txt

-This is a testharness.js-based test.
-PASS Property scroll-margin-block-end has initial value 0px
-PASS Property scroll-margin-block-end does not inherit
-PASS Property scroll-margin-block-start has initial value 0px
-PASS Property scroll-margin-block-start does not inherit
-PASS Property scroll-margin-bottom has initial value 0px
-PASS Property scroll-margin-bottom does not inherit
-PASS Property scroll-margin-inline-end has initial value 0px
-PASS Property scroll-margin-inline-end does not inherit
-PASS Property scroll-margin-inline-start has initial value 0px
-PASS Property scroll-margin-inline-start does not inherit
-PASS Property scroll-margin-left has initial value 0px
-PASS Property scroll-margin-left does not inherit
-PASS Property scroll-margin-right has initial value 0px
-PASS Property scroll-margin-right does not inherit
-PASS Property scroll-margin-top has initial value 0px
-PASS Property scroll-margin-top does not inherit
-PASS Property scroll-padding-block-end has initial value auto
-PASS Property scroll-padding-block-end does not inherit
-PASS Property scroll-padding-block-start has initial value auto
-PASS Property scroll-padding-block-start does not inherit
-PASS Property scroll-padding-bottom has initial value auto
-PASS Property scroll-padding-bottom does not inherit
-PASS Property scroll-padding-inline-end has initial value auto
-PASS Property scroll-padding-inline-end does not inherit
-PASS Property scroll-padding-inline-start has initial value auto
-PASS Property scroll-padding-inline-start does not inherit
-PASS Property scroll-padding-left has initial value auto
-PASS Property scroll-padding-left does not inherit
-PASS Property scroll-padding-right has initial value auto
-PASS Property scroll-padding-right does not inherit
-PASS Property scroll-padding-top has initial value auto
-PASS Property scroll-padding-top does not inherit
-PASS Property scroll-snap-align has initial value none
-PASS Property scroll-snap-align does not inherit
-PASS Property scroll-snap-stop has initial value normal
-PASS Property scroll-snap-stop does not inherit
-PASS Property scroll-snap-type has initial value none
-FAIL Property scroll-snap-type does not inherit assert_equals: expected "inline" but got "inline proximity"
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/css/css-scroll-snap/parsing/scroll-snap-type-computed-expected.txt

-This is a testharness.js-based test.
-PASS Property scroll-snap-type value 'none' computes to 'none'
-FAIL Property scroll-snap-type value 'x' computes to 'x' assert_equals: expected "x" but got "x proximity"
-FAIL Property scroll-snap-type value 'y' computes to 'y' assert_equals: expected "y" but got "y proximity"
-FAIL Property scroll-snap-type value 'block' computes to 'block' assert_equals: expected "block" but got "block proximity"
-FAIL Property scroll-snap-type value 'inline' computes to 'inline' assert_equals: expected "inline" but got "inline proximity"
-FAIL Property scroll-snap-type value 'both' computes to 'both' assert_equals: expected "both" but got "both proximity"
-PASS Property scroll-snap-type value 'y mandatory' computes to 'y mandatory'
-FAIL Property scroll-snap-type value 'inline proximity' computes to 'inline' assert_equals: expected "inline" but got "inline proximity"
-Harness: the test ran to completion.
-

third_party/blink/web_tests/external/wpt/css/css-typed-om/the-stylepropertymap/properties/scroll-snap-type-expected.txt

@@ -2,11 +2,11 @@ This is a testharness.js-based test.
 PASS Can set 'scroll-snap-type' to CSS-wide keywords
 PASS Can set 'scroll-snap-type' to var() references
 PASS Can set 'scroll-snap-type' to the 'none' keyword
-FAIL Can set 'scroll-snap-type' to the 'x' keyword assert_equals: expected "CSSKeywordValue" but got "CSSStyleValue"
-FAIL Can set 'scroll-snap-type' to the 'y' keyword assert_equals: expected "CSSKeywordValue" but got "CSSStyleValue"
-FAIL Can set 'scroll-snap-type' to the 'block' keyword assert_equals: expected "CSSKeywordValue" but got "CSSStyleValue"
-FAIL Can set 'scroll-snap-type' to the 'inline' keyword assert_equals: expected "CSSKeywordValue" but got "CSSStyleValue"
-FAIL Can set 'scroll-snap-type' to the 'both' keyword assert_equals: expected "CSSKeywordValue" but got "CSSStyleValue"
+PASS Can set 'scroll-snap-type' to the 'x' keyword
+PASS Can set 'scroll-snap-type' to the 'y' keyword
+PASS Can set 'scroll-snap-type' to the 'block' keyword
+PASS Can set 'scroll-snap-type' to the 'inline' keyword
+PASS Can set 'scroll-snap-type' to the 'both' keyword
 PASS Setting 'scroll-snap-type' to a length throws TypeError
 PASS Setting 'scroll-snap-type' to a percent throws TypeError
 PASS Setting 'scroll-snap-type' to a time throws TypeError