www/webkit-gtk*: unbreak with ICU 59.1

Source/JavaScriptCore/API/JSStringRef.cpp:40:12: error: no matching function for call to 'create'

return OpaqueJSString::create(chars, numChars).leakRef();
       ^~~~~~~~~~~~~~~~~~~~~~

./Source/JavaScriptCore/API/OpaqueJSString.h:44:39: note: candidate function not viable: no known conversion from 'const JSChar *' (aka 'const unsigned short *') to 'const LChar *' (aka 'const unsigned char *') for 1st argument

static PassRefPtr<OpaqueJSString> create(const LChar* characters, unsigned length)
                                  ^

./Source/JavaScriptCore/API/OpaqueJSString.h:49:39: note: candidate function not viable: no known conversion from 'const JSChar *' (aka 'const unsigned short *') to 'const UChar *' (aka 'const char16_t *') for 1st argument

static PassRefPtr<OpaqueJSString> create(const UChar* characters, unsigned length)
                                  ^

./Source/JavaScriptCore/API/OpaqueJSString.h:39:39: note: candidate function not viable: requires 0 arguments, but 2 were provided

static PassRefPtr<OpaqueJSString> create()
                                  ^

./Source/JavaScriptCore/API/OpaqueJSString.h:54:57: note: candidate function not viable: requires 1 argument, but 2 were provided

JS_EXPORT_PRIVATE static PassRefPtr<OpaqueJSString> create(const String&);
                                                    ^

Source/JavaScriptCore/API/JSStringRef.cpp:65:35: error: no matching function for call to 'createWithoutCopying'

return OpaqueJSString::create(StringImpl::createWithoutCopying(chars, numChars)).leakRef();
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

./Source/WTF/wtf/text/StringImpl.h:393:54: note: candidate function not viable: no known conversion from 'const JSChar *' (aka 'const unsigned short *') to 'const UChar *' (aka 'const char16_t *') for 1st argument

WTF_EXPORT_STRING_API static PassRef<StringImpl> createWithoutCopying(const UChar* characters, unsigned length);
                                                 ^

./Source/WTF/wtf/text/StringImpl.h:394:54: note: candidate function not viable: no known conversion from 'const JSChar *' (aka 'const unsigned short *') to 'const LChar *' (aka 'const unsigned char *') for 1st argument

WTF_EXPORT_STRING_API static PassRef<StringImpl> createWithoutCopying(const LChar* characters, unsigned length);
                                                 ^

Source/JavaScriptCore/API/JSStringRef.cpp:86:12: error: cannot initialize return object of type 'const JSChar *' (aka 'const unsigned short *') with an rvalue of type 'const UChar *' (aka 'const char16_t *')

return string->characters();
       ^~~~~~~~~~~~~~~~~~~~

PR: 218788
Obtained from: upstream (rebased)