Bug 1294645 - Don't use NS_CALLBACK for callbacks in nsI{Input,Output,UnicharInput},Stream.idl. r=froydnj.

Slightly less than half (93 / 210) of the NS_METHOD instances in the codebase
are because of the use of NS_CALLBACK in
nsI{Input,Output,UnicharInput},Stream.idl. The use of __stdcall on Win32 isn't
important for these callbacks because they are only used as arguments to
[noscript] methods.

This patch converts them to vanilla |nsresult| functions. It increases the size
of xul.dll by about ~600 bytes, which is about 0.001%.

docshell/base/nsDocShell.cpp

@@ -11111,7 +11111,7 @@ nsDocShell::DoURILoad(nsIURI* aURI,
   return rv;
 }
 
-static NS_METHOD
+static nsresult
 AppendSegmentToString(nsIInputStream* aIn,
                       void* aClosure,
                       const char* aFromRawSegment,