Call Element::rebuildLayoutTree from Document::updateStyle directly

third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp

Index: third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp
diff --git a/third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp b/third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp
index e29035dc3be460bdb1f41826b77722664ae310a0..5c4e891dbb32a235949d9a39a40c4e09b25971f3 100644
--- a/third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp
+++ b/third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp
@@ -82,10 +82,10 @@ ShadowRoot& ElementShadow::addShadowRoot(Element& shadowHost,
   setNeedsDistributionRecalc();
 
   shadowRoot->insertedInto(&shadowHost);
-  shadowHost.setChildNeedsStyleRecalc();

[esprehn, 2017/02/16 21:24:33]

I think we probably want to leave this, while technically it's not needed, it
preserves the invariant that if the shaodwRoot needs a recalc (which it does
since it's brand new), that the host has the child bit set.

(I know this was my suggestion.)

[nainar, 2017/02/16 22:26:09]

Sure :)

   shadowHost.setNeedsStyleRecalc(
       SubtreeStyleChange,
       StyleChangeReasonForTracing::create(StyleChangeReason::Shadow));
+  shadowRoot->setNeedsReattachLayoutTree();
 
   InspectorInstrumentation::didPushShadowRoot(&shadowHost, shadowRoot);