[blink]: removes WebkitBoxLayoutUsesFlexLayout feature

It's the default and we started removing code that means we can no
longer disable it.

BUG=993813
TEST=none

Change-Id: I9f65953bd5a696ad9281644285caa3f9ccba8c81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975303
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726396}

third_party/blink/renderer/core/style/computed_style.cc

@@ -220,8 +220,7 @@ bool ComputedStyle::NeedsReattachLayoutTree(const ComputedStyle* old_style,
   // line-clamping is currently only handled by LayoutDeprecatedFlexibleBox,
   // so that if line-clamping changes then the LayoutObject needs to be
   // recreated.
-  if (RuntimeEnabledFeatures::WebkitBoxLayoutUsesFlexLayoutEnabled() &&
-      (new_style->IsDeprecatedWebkitBox()) &&
+  if (new_style->IsDeprecatedWebkitBox() &&
       (old_style->HasLineClamp() != new_style->HasLineClamp() &&
        new_style->BoxOrient() == EBoxOrient::kVertical)) {
     return true;

third_party/blink/renderer/core/style/computed_style.h

@@ -1223,7 +1223,6 @@ class ComputedStyle : public ComputedStyleBase,
   }
   bool IsDeprecatedFlexboxUsingFlexLayout() const {
     return IsDeprecatedWebkitBox() &&
-           RuntimeEnabledFeatures::WebkitBoxLayoutUsesFlexLayoutEnabled() &&
            (!HasLineClamp() || BoxOrient() == EBoxOrient::kHorizontal);
   }
 

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1829,10 +1829,6 @@
       name: "WebHID",
       status: "experimental",
     },
-    {
-      name: "WebkitBoxLayoutUsesFlexLayout",
-      status: "stable",
-    },
     {
       name: "WebNFC",
       status: "experimental",