Remove flag for CSS Alt Text

Enabled by default in stable for awhile now, so safe to remove.

BUG=860016

Change-Id: I2deea943849ce11a3db2a98e85df6c6aee5d38cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850689Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705811}

third_party/blink/renderer/core/css/properties/longhands/longhands_custom.cc

@@ -1899,8 +1899,7 @@ const CSSValue* Content::ParseSingleValue(CSSParserTokenRange& range,
       }
     }
     if (!parsed_value) {
-      if (RuntimeEnabledFeatures::CSSAltTextEnabled() &&
-          css_property_parser_helpers::ConsumeSlashIncludingWhitespace(range)) {
+      if (css_property_parser_helpers::ConsumeSlashIncludingWhitespace(range)) {
         // No values were parsed before the slash, so nothing to apply the
         // alternative text to.
         if (!values->length())

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -369,10 +369,6 @@
       depends_on: ["StackedCSSPropertyAnimations"],
       status: "experimental",
     },
-    {
-      name: "CSSAltText",
-      status: "stable",
-    },
     {
       name: "CSSBackdropFilter",
       status: "stable",

third_party/blink/web_tests/accessibility/css-alt-text.html

@@ -15,10 +15,6 @@
 
 <script>
 
-test(function(t) {
-  assert_true(internals.runtimeFlags.cssAltTextEnabled);
-}, "Make sure that CSS Alt Text is enabled.");
-
 test(function(t) {
   var parentAXNode = accessibilityController.accessibleElementById("controlparent");
   var contentTextNode = parentAXNode.childAtIndex(0);