Use EditingTestBase for PositionTest

Source/core/editing/EditingTestBase.cpp

Index: Source/core/editing/EditingTestBase.cpp
diff --git a/Source/core/editing/EditingTestBase.cpp b/Source/core/editing/EditingTestBase.cpp
index d22b8d7e0fb046feb7439be5d14418472fa7a593..356434e2f03d06fb4bb8eb48e50e4ffe3ece2d8b 100644
--- a/Source/core/editing/EditingTestBase.cpp
+++ b/Source/core/editing/EditingTestBase.cpp
@@ -45,7 +45,9 @@ void EditingTestBase::setBodyContent(const char* bodyContent)
 
 PassRefPtrWillBeRawPtr<ShadowRoot> EditingTestBase::setShadowContent(const char* shadowContent)
 {
-    return createShadowRootForElementWithIDAndSetInnerHTML(document(), "host", shadowContent);
+    RefPtrWillBeRawPtr<ShadowRoot> shadowRoot = createShadowRootForElementWithIDAndSetInnerHTML(document(), "host", shadowContent);
+    document().recalcDistribution();
+    return shadowRoot.release();
 }
 
 Position EditingTestBase::positionInDOMTree(Node& anchor, int offset)