Pdfviewer: generate isAFoo() and getAsFoo() api for fields that can have multiple types.

experimental/PdfViewer/datatypes.py

Index: experimental/PdfViewer/datatypes.py
===================================================================
--- experimental/PdfViewer/datatypes.py	(revision 9642)
+++ experimental/PdfViewer/datatypes.py	(working copy)
@@ -45,4 +45,30 @@
 class CppNull:
   def toCpp(self):
     return 'NULL'
+    
+class PdfDateNever:
+  def toCpp(self):
+    return 'SkPdfDate()'
+
+class FileSpecNone:
+  def toCpp(self):
+    return 'SkPdfFileSpec()'
+
+class PdfEmptyRect:
+  def toCpp(self):
+    return 'SkRect()'
+    
+class PdfEmptyStream:
+  def toCpp(self):
+    return 'SkPdfStream()'
+    
+class PdfArrayNone:
+  def toCpp(self):
+    return 'SkPdfArray()'
+    
+class PdfFunctionNone:
+  def toCpp(self):
+    return 'SkPdfFunction()'
+    
+