Add DCHECK in copy constructor of StyleVisualData for m_zoom.

third_party/WebKit/Source/core/style/StyleVisualData.cpp

Index: third_party/WebKit/Source/core/style/StyleVisualData.cpp
diff --git a/third_party/WebKit/Source/core/style/StyleVisualData.cpp b/third_party/WebKit/Source/core/style/StyleVisualData.cpp
index c4ea3d0f720a0a5eb5887a2e6bf8138b6577a708..55a9a72afeab31347b540c1f6fd396bfc366d1b1 100644
--- a/third_party/WebKit/Source/core/style/StyleVisualData.cpp
+++ b/third_party/WebKit/Source/core/style/StyleVisualData.cpp
@@ -37,6 +37,8 @@ StyleVisualData::StyleVisualData(const StyleVisualData& o)
       clip(o.clip),
       hasAutoClip(o.hasAutoClip),
       textDecoration(o.textDecoration),
-      m_zoom(ComputedStyle::initialZoom()) {}
+      m_zoom(ComputedStyle::initialZoom()) {
+  DCHECK(m_zoom == o.m_zoom);  // crbug.com/689330
+}
 
 }  // namespace blink