Faster WTF::toASCIILower() implementation for 8-bit strings

Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.h',
             'AddressSanitizer.h',
             'AddressSpaceRandomization.cpp',
             'AddressSpaceRandomization.h',
             'Alignment.h',
             'ArrayBuffer.cpp',
@@ skipping 193 matching lines @@
             'unicode/CharacterNames.h',
             'unicode/Collator.h',
             'unicode/UTF8.cpp',
             'unicode/UTF8.h',
             'unicode/Unicode.h',
             'unicode/icu/CollatorICU.cpp',
             'unicode/icu/UnicodeIcu.h',
         ],
         'wtf_unittest_files': [
             'ArrayBufferBuilderTest.cpp',
+            'ASCIICTypeTest.cpp',
             'CheckedArithmeticTest.cpp',
             'DequeTest.cpp',
             'DoubleBufferedDequeTest.cpp',
             'FunctionalTest.cpp',
             'HashMapTest.cpp',
             'HashSetTest.cpp',
             'ListHashSetTest.cpp',
             'MathExtrasTest.cpp',
             'OptionalTest.cpp',
             'PartitionAllocTest.cpp',
@@ skipping 15 matching lines @@
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
         ],
         'wtf_unittest_helper_files': [
             'testing/WTFTestHelpers.cpp',
             'testing/WTFTestHelpers.h',
             'testing/WTFUnitTestHelpersExport.h',
         ],
     },
 }