IDL compiler: mostly merge Non-wrapper types and RefPtr conversion exceptions
IDL compiler: mostly merge Non-wrapper types and RefPtr conversion exceptions

This makes no changes to generated code, and
reduces the special cases to 2 types:
DOMStringList and XPathNSResolver,
each used a handful of places.

Uses:

DOMStringList:
Location.idl
SecurityPolicy.idl
IDBDatabase.idl
IDBObjectStore.idl

XPathNSResolver:
DocumentXPathEvaluator.idl (partial for Document)
XPathEvaluator.idl


Background:
In the rewrite, I noticed that the list of exception in GetNativeType
(where the native Blink C++ type is |RefPtr<$idlType>|)
largely overlapped with nonWrapperTypes, and haraken requested that I
merge these if possible.

BUG=239771

Review URL: https://codereview.chromium.org/28863007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@160010 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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