Bug 1333183 - Prime the root element cache before the servo traversal. r=emilio

dom/base/nsDocument.cpp

@@ -4037,6 +4037,10 @@ nsIDocument::GetRootElement() const
 Element*
 nsDocument::GetRootElementInternal() const
 {
+  // We invoke GetRootElement() immediately before the servo traversal, so we
+  // should always have a cache hit from Servo.
+  MOZ_ASSERT(NS_IsMainThread());
+
   // Loop backwards because any non-elements, such as doctypes and PIs
   // are likely to appear before the root element.
   uint32_t i;

layout/style/ServoStyleSet.cpp

@@ -192,6 +192,11 @@ ServoStyleSet::PrepareAndTraverseSubtree(RawGeckoElementBorrowed aRoot,
                                          mozilla::TraversalRootBehavior aRootBehavior) {
   ResolveMappedAttrDeclarationBlocks();
 
+  // Get the Document's root element to ensure that the cache is valid before
+  // calling into the (potentially-parallel) Servo traversal, where a cache hit
+  // is necessary to avoid a data race when updating the cache.
+  mozilla::Unused << aRoot->OwnerDoc()->GetRootElement();
+
   MOZ_ASSERT(!sInServoTraversal);
   sInServoTraversal = true;
   Servo_TraverseSubtree(aRoot, mRawSet.get(), aRootBehavior);