AU: When applying delta locally, get source checksums

When applying a delta on the host, we need to calculate the original
checksums for the rootfs/kernel for the delta performer. This CL does
that.

Also:
- Log old/new checksums in performer
- Add base64 encode function to OmahaHashCalculator

BUG=8062
TEST=unittests; applied delta on host

Review URL: http://codereview.chromium.org/4042004

Change-Id: I953d7dd34cb65269e9b44e22c87a13a7f52e66d4
diff --git a/omaha_hash_calculator.h b/omaha_hash_calculator.h
index dc9fe4e..762b8d4 100644
--- a/omaha_hash_calculator.h
+++ b/omaha_hash_calculator.h
@@ -70,6 +70,8 @@
   static std::string OmahaHashOfString(const std::string& str);
   static std::string OmahaHashOfData(const std::vector<char>& data);
 
+  static bool Base64Encode(const void* data, size_t size, std::string* out);
+
  private:
   // If non-empty, the final base64 encoded hash and the raw hash. Will only be
   // set to non-empty when Finalize is called.