update_engine: replace std::vector<char> with chromeos::Blob

To make update engine consistent with the rest of platform2 code
replaced std::vector<char> as the container of binary data with
chromeos::Blob.

BUG=None
TEST=`FEATURES=test emerge-link update_engine`

Change-Id: I6385fd2257d15aa24bfa74ac35512c2a06c33012
Reviewed-on: https://chromium-review.googlesource.com/247793
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/download_action.h b/download_action.h
index 3cf7e6a..52efb46 100644
--- a/download_action.h
+++ b/download_action.h
@@ -70,7 +70,7 @@
 
   // HttpFetcherDelegate methods (see http_fetcher.h)
   void ReceivedBytes(HttpFetcher *fetcher,
-                     const char* bytes, int length) override;
+                     const void* bytes, size_t length) override;
   void SeekToOffset(off_t offset) override;
   void TransferComplete(HttpFetcher *fetcher, bool successful) override;
   void TransferTerminated(HttpFetcher *fetcher) override;
@@ -103,7 +103,7 @@
   //
   // This method does nothing if SetupP2PSharingFd() hasn't been
   // called or if CloseP2PSharingFd() has been called.
-  void WriteToP2PFile(const char *data, size_t length, off_t file_offset);
+  void WriteToP2PFile(const void *data, size_t length, off_t file_offset);
 
   // The InstallPlan passed in
   InstallPlan install_plan_;