Minor clean-ups.

* Track the length of the dex file in RawDexFile.  This can be used to
  improve its assertions.

* Move a sanity check for MmapCloser into the constructor.

* Add a TODO for moving the base64 openers into test code.

* Make the base64 decoder assume C strings, mention this in a comment.

Change-Id: Ib4d5a43d23ebe8819032763c42c536c8b826723a
diff --git a/src/base64.h b/src/base64.h
index 320148d..160046d 100644
--- a/src/base64.h
+++ b/src/base64.h
@@ -7,7 +7,8 @@
 
 namespace art {
 
-byte* DecodeBase64(const char* src, size_t size, size_t* dst_size);
+// Decodes a C string with base64 encoded data.
+byte* DecodeBase64(const char* src, size_t* dst_size);
 
 }  // namespace art