[PE]: Remove LayoutViewIsolationNodes flag which has been on for a while.

This patch removes the flag added earlier, since the feature has been
on without any problems.

R=chrishtr@chromium.org, pdr@chromium.org

Change-Id: I42e41417f531e2ba47a096cf172234d4f549a5fd
Reviewed-on: https://chromium-review.googlesource.com/c/1394915Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619963}

third_party/blink/renderer/core/paint/paint_property_tree_builder.cc

@@ -322,8 +322,7 @@ static bool NeedsIsolationNodes(const LayoutObject& object) {
 
   // Layout view establishes isolation with the exception of local roots (since
   // they are already essentially isolated).
-  if (RuntimeEnabledFeatures::LayoutViewIsolationNodesEnabled() &&
-      object.IsLayoutView()) {
+  if (object.IsLayoutView()) {
     const auto* parent_frame = object.GetFrame()->Tree().Parent();
     return parent_frame && parent_frame->IsLocalFrame();
   }

third_party/blink/renderer/core/paint/paint_property_tree_builder_test.cc

@@ -1857,9 +1857,6 @@ TEST_P(PaintPropertyTreeBuilderTest, TransformNodesAcrossSubframes) {
 }
 
 TEST_P(PaintPropertyTreeBuilderTest, FramesEstablishIsolation) {
-  if (!RuntimeEnabledFeatures::LayoutViewIsolationNodesEnabled())
-    return;
-
   SetBodyInnerHTML(R"HTML(
     <style>
       body { margin: 0; }

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -694,10 +694,6 @@
       name: "LayoutNGFragmentCaching",
       implied_by: ["LayoutNG"],
     },
-    {
-      name: "LayoutViewIsolationNodes",
-      status: "stable",
-    },
     {
       name: "LazyFrameLoading",
     },