AU: Verify source rootfs/kernel hashes before applying delta.

New style full updates will not send the old rootfs hash so no check takes
place.

BUG=7562
TEST=unit tests, gmerged on device and tested with good/bad source partition

Change-Id: I65b28bf57110e4d87472d4aea59121878cde24b0

Review URL: http://codereview.chromium.org/3712003
diff --git a/omaha_hash_calculator.h b/omaha_hash_calculator.h
index 5daba78..dc9fe4e 100644
--- a/omaha_hash_calculator.h
+++ b/omaha_hash_calculator.h
@@ -62,6 +62,8 @@
 
   static bool RawHashOfData(const std::vector<char>& data,
                             std::vector<char>* out_hash);
+  static off_t RawHashOfFile(const std::string& name, off_t length,
+                             std::vector<char>* out_hash);
 
   // Used by tests
   static std::string OmahaHashOfBytes(const void* data, size_t length);