Ship CSS comparison functions min, max and clamp

Current progress:
- Implementation finished
- Test suite finished except for a minor serialization spec issue [1],
  which seems minor enough not to block the launch
- The spec issue with table layout [2] is already resolved. In case of
  similar spec issues in other layout algorithms, I believe a similar
  approach (UA must treat the value as auto) can be adopted.

Therefore, I believe there is no longer any issue blocking the shipping
of the feature.

[1] https://github.com/w3c/csswg-drafts/issues/4372
[2] https://github.com/w3c/csswg-drafts/issues/94

Bug: 825895
Change-Id: I0fe8e934d4429fa6ed1c3dd8847f70b1ba350e07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829471Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701148}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -385,7 +385,7 @@
       // CSS min(), max() and clamp()
       // https://drafts.csswg.org/css-values-4/#comp-func
       name: "CSSComparisonFunctions",
-      status: "test",
+      status: "stable",
     },
     {
       // Support for CSS content-size property.