Selection API: Add diagnostic code for extend() crash

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

Index: third_party/WebKit/Source/core/editing/DOMSelection.cpp
diff --git a/third_party/WebKit/Source/core/editing/DOMSelection.cpp b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
index bb6e173919ba4199fe6b7bb9a67fccceaed84d45..ae438372f5031fe19ac97d7639391e87a3002aa9 100644
--- a/third_party/WebKit/Source/core/editing/DOMSelection.cpp
+++ b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
@@ -459,6 +459,17 @@ void DOMSelection::extend(Node* node,
   // 3. Let oldAnchor and oldFocus be the context object's anchor and focus, and
   // let newFocus be the boundary point (node, offset).
   const Position& oldAnchor = anchorPosition();
+  // TODO(tkent): Diagnostic checks for crbug.com/693578.  They should be
+  // removed before M58 branch.
+  if (oldAnchor.isNull()) {
+    if (Range* range = documentCachedRange()) {
+      LOG(FATAL)
+          << "Selection has a cached Range, but anchorPosition is null. start="
+          << range->startContainer() << " end=" << range->endContainer();
+    } else if (frame() && !frame()->selection().isNone()) {
+      LOG(FATAL) << "FrameSelection is not none, but anchorPosition is null.";
+    }
+  }
   DCHECK(!oldAnchor.isNull());
   const Position newFocus(node, offset);