Returning early when setting a frame name to the same name.

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 827cbb629e79a1ef1051c50fd41c00950a8d6f15..4b6e9d17b45d0d3ea6719c58cd470ee2c0147b38 100644
--- a/third_party/WebKit/Source/core/page/FrameTree.cpp
+++ b/third_party/WebKit/Source/core/page/FrameTree.cpp
@@ -54,6 +54,20 @@ FrameTree::~FrameTree()
 
 void FrameTree::setName(const AtomicString& name)
 {
+    // Do not recalculate m_uniqueName if there is no real change of m_name.
+    // This is not just a performance optimization - other code relies on the
+    // assumption that unique name shouldn't change if the assigned name didn't
+    // change (i.e. code in content::FrameTreeNode::SetFrameName).
+    if (m_name == name) {
+        // Assert that returning early below won't leave m_uniqueName in an
+        // uninitialized state - it should only be null if m_name is also
+        // null and only if it is for the main frame.
+        // m_uniqueName.isNull() should imply m_name.isNull() && !parent().
+        // this FrameTree is for a main frame.
+        DCHECK(!m_uniqueName.isNull() || (m_name.isNull() && !parent()));
+        return;
+    }
+
     m_name = name;
 
     // Remove our old frame name so it's not considered in calculateUniqueNameForChildFrame