Fixed crash by replacing DCHECK with returning nullptr.

third_party/WebKit/Source/core/dom/NodeComputedStyle.h

Index: third_party/WebKit/Source/core/dom/NodeComputedStyle.h
diff --git a/third_party/WebKit/Source/core/dom/NodeComputedStyle.h b/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
index a5a9f39ea3c6d12e0662013b8a89dd45807f95d0..d7345264df9b5a75a8206a69643b3dba166c56d9 100644
--- a/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
+++ b/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
@@ -52,7 +52,8 @@ inline ComputedStyle* Node::mutableComputedStyle() const
         return nonLayoutObjectComputedStyle();
     if (hasRareData()) {
         NodeRareData* rareData = this->rareData();
-        DCHECK(rareData->isElementRareData());
+        if (!rareData->isElementRareData())
+            return nullptr;
         return static_cast<ElementRareData*>(rareData)->computedStyle();
     }
     return m_data.m_computedStyle;