Ensure valid VisiblePosition input for inSameBlock

third_party/WebKit/Source/core/editing/VisibleUnits.cpp

Index: third_party/WebKit/Source/core/editing/VisibleUnits.cpp
diff --git a/third_party/WebKit/Source/core/editing/VisibleUnits.cpp b/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
index d5e2463f6bc0514c384c714ae2689d8ed1eaef36..5a9dda91338e369e5a20e3c4d6e43df14c3f4b9e 100644
--- a/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
+++ b/third_party/WebKit/Source/core/editing/VisibleUnits.cpp
@@ -1923,8 +1923,8 @@ VisiblePosition endOfBlock(const VisiblePosition& visiblePosition, EditingBounda
 
 bool inSameBlock(const VisiblePosition& a, const VisiblePosition& b)
 {
-    // TODO(xiaochengh): Ensure that this function is called with valid |a| and
-    // |b|, and add |DCHECK(a.isValid())| and |DCHECK(b.isValid())|
+    DCHECK(a.isValid()) << a;
+    DCHECK(b.isValid()) << b;
     return !a.isNull() && enclosingBlock(a.deepEquivalent().computeContainerNode()) == enclosingBlock(b.deepEquivalent().computeContainerNode());
 }