Fix RefPtr's move assignment operator

Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.h',
             'AddressSpaceRandomization.cpp',
             'AddressSpaceRandomization.h',
             'Alignment.h',
             'ArrayBuffer.cpp',
             'ArrayBuffer.h',
@@ skipping 198 matching lines @@
         'wtf_unittest_files': [
             'ArrayBufferBuilderTest.cpp',
             'CheckedArithmeticTest.cpp',
             'DequeTest.cpp',
             'FunctionalTest.cpp',
             'HashMapTest.cpp',
             'HashSetTest.cpp',
             'ListHashSetTest.cpp',
             'MathExtrasTest.cpp',
             'PartitionAllocTest.cpp',
+            'RefPtrTest.cpp',
             'SaturatedArithmeticTest.cpp',
             'StringExtrasTest.cpp',
             'StringHasherTest.cpp',
             'TemporaryChangeTest.cpp',
             'TreeNodeTest.cpp',
             'VectorTest.cpp',
             'testing/WTFTestHelpersTest.cpp',
             'text/AtomicStringTest.cpp',
             'text/CStringTest.cpp',
             'text/StringBuilderTest.cpp',
             'text/StringImplTest.cpp',
             'text/StringOperatorsTest.cpp',
             'text/TextCodecReplacementTest.cpp',
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
         ],
         'wtf_unittest_helper_files': [
             'testing/WTFTestHelpers.cpp',
             'testing/WTFTestHelpers.h',
             'testing/WTFUnitTestHelpersExport.h',
         ],
     },
 }