Update XMLHttpRequest to use enum class instead of string for enumeration

Update XMLHttpRequest to use enum class instead of string for enumeration
https://bugs.webkit.org/show_bug.cgi?id=157238

Reviewed by Chris Dumez.

* bindings/js/JSXMLHttpRequestCustom.cpp:
(WebCore::JSXMLHttpRequest::response): Rewrite to use responseType function
instead of responseTypeCode function.

* bindings/scripts/CodeGenerator.pm: Removed XMLHttpRequestResponseType
from the list of enumerations that use a string-based implementation.

* bindings/scripts/CodeGeneratorJS.pm:
(GetEnumerationImplementationContent): Use emptyString instead of ASCIILiteral
when the string constant is empty, to avoid the assertion.

* xml/XMLHttpRequest.cpp:
(WebCore::XMLHttpRequest::XMLHttpRequest): Move simple scalar data member
initialization to where the data members are defined in the header.
(WebCore::XMLHttpRequest::responseText): Use m_responseType instead of
m_responseTypeCode.
(WebCore::XMLHttpRequest::didCacheResponseJSON): Ditto.
(WebCore::XMLHttpRequest::responseXML): Ditto.
(WebCore::XMLHttpRequest::responseBlob): Ditto.
(WebCore::XMLHttpRequest::responseArrayBuffer): Ditto.
(WebCore::XMLHttpRequest::setResponseType): Ditto.
(WebCore::XMLHttpRequest::responseType): Moved to header now that it's a
trivial function that should be inlined.
(WebCore::XMLHttpRequest::open): Use m_responseType instead of
m_responseTypeCode.
(WebCore::XMLHttpRequest::responseMIMEType): Removed one redundant check.
(WebCore::XMLHttpRequest::status): Ditto.
(WebCore::XMLHttpRequest::statusText): Ditto.
(WebCore::shouldDecodeResponse): Moved here from the header. This is now
a non-member function that gets passed a response type. It uses a switch
now instead of ordering the types so that the binary ones are all at the
end of the list.
(WebCore::XMLHttpRequest::didReceiveData): Use the new shouldDecodeResponse.
Also removed an unnecessary check that non-decoded responses are of type
"arraybuffer" or "blob"; that's the same check that shouldDecodeResponse does.

* xml/XMLHttpRequest.h: Removed some unneeded includes and forward declarations.
Removed a comment about the State values needing to be stable numbers; for one
thing the bindings already check that these values have no changed. It's not a
valuable comment. Removed ResponseTypeCode, responseTypeCode, and
m_responseTypeCode. Changed the type of the result of responseType and the
argument to setResponseType from a string to XMLHttpRequestResponseType.
Removed shouldDecodeResponse. Added the initial values for lots of data members
that are simple scalars, matching what was set in the constructor before.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@200300 268f45cc-cd09-0410-ab3c-d52691b4dbfc