Main frame's unique name can always be null.

third_party/WebKit/Source/core/page/FrameTree.cpp

Index: third_party/WebKit/Source/core/page/FrameTree.cpp
diff --git a/third_party/WebKit/Source/core/page/FrameTree.cpp b/third_party/WebKit/Source/core/page/FrameTree.cpp
index e5b70f736e45c19dd7c395c724e3aa29e60fc621..5539d1d900a10f7ed910443dba9c091217872fee 100644
--- a/third_party/WebKit/Source/core/page/FrameTree.cpp
+++ b/third_party/WebKit/Source/core/page/FrameTree.cpp
@@ -78,29 +78,30 @@ void FrameTree::setName(const AtomicString& name)
     if (toLocalFrame(m_thisFrame)->loader().stateMachine()->committedFirstRealDocumentLoad())
         return;
 
-    // Remove our old frame name so it's not considered in calculateUniqueNameForChildFrame
-    // and appendUniqueSuffix calls below.
+    // Leave main frame's unique name set to a null string.
+    if (!parent())
+        return;
+
+    // Remove our old frame name so it's not considered in
+    // calculateUniqueNameForChildFrame call below.
     m_uniqueName = AtomicString();
 
     // Calculate a new unique name based on inputs.
-    if (parent()) {
-        setUniqueName(
-            parent()->tree().calculateUniqueNameForChildFrame(m_thisFrame, name, nullAtom));
-    } else if (name.isEmpty() || !uniqueNameExists(name)) {
-        // Only main frame can have an empty unique name, so for main frames
-        // emptiness guarantees uniquness.
-        setUniqueName(name);
-    } else {
-        setUniqueName(appendUniqueSuffix(name, "<!--framePosition"));
-    }
+    setUniqueName(
+        parent()->tree().calculateUniqueNameForChildFrame(m_thisFrame, name, nullAtom));
 }
 
 void FrameTree::setPrecalculatedName(const AtomicString& name, const AtomicString& uniqueName)
 {
     m_name = name;
 
-    // Non-main frames should have a non-empty unique name.
-    DCHECK(!parent() || !uniqueName.isEmpty());
+    if (parent()) {
+        // Non-main frames should have a non-empty unique name.
+        DCHECK(!uniqueName.isEmpty());
+    } else {
+        // Unique name of main frames should always stay empty.
+        DCHECK(uniqueName.isEmpty());
+    }
 
     // TODO(lukasza): We would like to assert uniqueness below (i.e. by calling
     // setUniqueName), but
@@ -313,10 +314,9 @@ AtomicString FrameTree::calculateUniqueNameForChildFrame(
     // backcompatibility-aware approach has resulted so far in the following
     // rather baroque format... :
     //
-    //   uniqueName ::= <assignedName> | <generatedName>
+    //   uniqueName ::= <nullForMainFrame> | <assignedName> | <generatedName>
     //                 (generatedName is used if assignedName is
-    //                 1) non-unique / conflicts with other frame's unique name or
-    //                 2) assignedName is empty for a non-main frame)
+    //                 non-unique / conflicts with other frame's unique name.
     //
     //   assignedName ::= value of iframe's name attribute
     //                 or value assigned to window.name (*before* the first
@@ -327,9 +327,7 @@ AtomicString FrameTree::calculateUniqueNameForChildFrame(
     //                  not unique after all)
     //
     //   oldGeneratedName ::= "<!--framePath //" ancestorChain "/<!--frame" childCount "-->-->"
-    //                  (main frame is special - oldGeneratedName for main frame
-    //                  is always the frame's assignedName;  oldGeneratedName is
-    //                  generated by generateUniqueNameCandidate method).
+    //                  (oldGeneratedName is generated by generateUniqueNameCandidate method).
     //
     //   childCount ::= current number of siblings
     //
@@ -344,20 +342,17 @@ AtomicString FrameTree::calculateUniqueNameForChildFrame(
     //   newUniqueSuffix ::= "<!--framePosition" framePosition "/" retryNumber "-->"
     //
     //   framePosition ::= "-" numberOfSiblingsBeforeChild [ framePosition-forParent? ]
-    //                   | <empty string for main frame>
     //
     //   retryNumber ::= smallest non-negative integer resulting in unique name
 }
 
 void FrameTree::setUniqueName(const AtomicString& uniqueName)
 {
-    // Main frame is the only frame that can have an empty unique name.
-    if (parent()) {
-        DCHECK(!uniqueName.isEmpty() && !uniqueNameExists(uniqueName));
-    } else {
-        DCHECK(uniqueName.isEmpty() || !uniqueNameExists(uniqueName));
-    }
+    // Only subframes can have a non-null unique name - setUniqueName should
+    // only be called for subframes and never for a main frame.
+    DCHECK(parent());
 
+    DCHECK(!uniqueName.isEmpty() && !uniqueNameExists(uniqueName));
     m_uniqueName = uniqueName;
 }