Retire TextUnderlinePositionLeftRight flag

'left' and 'right' values for the 'text-underline-position' property
shipped in M71.
https://www.chromestatus.com/features/5749634488074240

The flag TextUnderlinePositionLeftRight has been enabled since
https://chromium-review.googlesource.com/1226485

BUG=313888

Change-Id: I893bbc0d402439a092c96d39b629dd8c94627622
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616832
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661316}
diff --git a/third_party/blink/renderer/core/css/properties/longhands/longhands_custom.cc b/third_party/blink/renderer/core/css/properties/longhands/longhands_custom.cc
index b31247f..5fe98bfb 100644
--- a/third_party/blink/renderer/core/css/properties/longhands/longhands_custom.cc
+++ b/third_party/blink/renderer/core/css/properties/longhands/longhands_custom.cc
@@ -6301,15 +6301,12 @@
 
   CSSIdentifierValue* under_value =
       css_property_parser_helpers::ConsumeIdent<CSSValueID::kUnder>(range);
-  CSSIdentifierValue* left_or_right_value = nullptr;
-  if (RuntimeEnabledFeatures::TextUnderlinePositionLeftRightEnabled()) {
-    left_or_right_value =
-        css_property_parser_helpers::ConsumeIdent<CSSValueID::kLeft,
-                                                  CSSValueID::kRight>(range);
-    if (left_or_right_value && !under_value) {
-      under_value =
-          css_property_parser_helpers::ConsumeIdent<CSSValueID::kUnder>(range);
-    }
+  CSSIdentifierValue* left_or_right_value =
+      css_property_parser_helpers::ConsumeIdent<CSSValueID::kLeft,
+                                                CSSValueID::kRight>(range);
+  if (left_or_right_value && !under_value) {
+    under_value =
+        css_property_parser_helpers::ConsumeIdent<CSSValueID::kUnder>(range);
   }
   if (!under_value && !left_or_right_value) {
     return nullptr;
diff --git a/third_party/blink/renderer/platform/runtime_enabled_features.json5 b/third_party/blink/renderer/platform/runtime_enabled_features.json5
index ecdf61a..87b4dfa 100644
--- a/third_party/blink/renderer/platform/runtime_enabled_features.json5
+++ b/third_party/blink/renderer/platform/runtime_enabled_features.json5
@@ -1415,10 +1415,6 @@
       status: "experimental",
     },
     {
-      name: "TextUnderlinePositionLeftRight",
-      status: "stable",
-    },
-    {
       name: "TimerThrottlingForBackgroundTabs",
       status: "stable",
     },