[css-values] Add a runtime flag for CSS comparison functions

Intent to implement and ship: http://bit.ly/2J75JAQ

Bug: 825895
Change-Id: Ib5ed82072e189f8c99f5ebc157127d389bf87375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678883
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672679}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -344,6 +344,11 @@
     {
       name: "CSSColorScheme",
     },
+    {
+      // CSS min(), max() and clamp()
+      // https://drafts.csswg.org/css-values-4/#comp-func
+      name: "CSSComparisonFunctions",
+    },
     {
       name: "CSSFocusVisible",
       status: "experimental",