Bug 1438700, part 2 - Deduplicate XPT strings. r=glandium

Various strings, like nsISupports, appear many times in XPT data. This
patch adds a cache so we don't write the same string multiple times.

MozReview-Commit-ID: 6buBrXwHqQz

--HG--
extra : rebase_source : 54dea83a9134710c5600828ab68ef3f935f46afd

xpcom/typelib/xpt/tools/xpt.py

@@ -577,10 +577,14 @@ class CachedStringWriter(object):
     def __init__(self, file, data_pool_offset):
         self.file = file
         self.data_pool_offset = data_pool_offset
+        self.names = {}
 
     def write(self, s):
         if s:
+            if s in self.names:
+                return self.names[s]
             offset = self.file.tell() - self.data_pool_offset + 1
+            self.names[s] = offset
             self.file.write(s + "\x00")
             return offset
         else: