Disable iframe isolation nodes via a flag.

This patch introduces a flag for layout view isolation nodes, since
it seems to have caused a memory regression. The flag would allow to
easily test the memory usage and other perf benefits. I'll remove the
flag and enable by default when the problems are resolved.

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

Bug: 895270
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iaa74c997dc3d211d0a870fdff303a9c94b5312e9
Reviewed-on: https://chromium-review.googlesource.com/c/1282204Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600036}

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

@@ -322,7 +322,8 @@ static bool NeedsIsolationNodes(const LayoutObject& object) {
 
   // Layout view establishes isolation with the exception of local roots (since
   // they are already essentially isolated).
-  if (object.IsLayoutView()) {
+  if (RuntimeEnabledFeatures::LayoutViewIsolationNodesEnabled() &&
+      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

@@ -1812,6 +1812,9 @@ 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

@@ -670,6 +670,9 @@
       name: "LayoutNGFragmentCaching",
       implied_by: ["LayoutNG"],
     },
+    {
+      name: "LayoutViewIsolationNodes",
+    },
     {
       name: "LazyFrameLoading",
     },