Support optional powerwash from Omaha response.

Will powerwash if 'Powerwash="true"' is set by Omaha.

Bug: 73969717
Test: update_engine_unittests
Change-Id: Ic7ba131f5c77cdb1ca28a69530f55bea4d27b0b5
diff --git a/omaha_response.h b/omaha_response.h
index b973eb5..3fb2f6d 100644
--- a/omaha_response.h
+++ b/omaha_response.h
@@ -72,6 +72,9 @@
   // True if the Omaha rule instructs us to disable p2p for sharing.
   bool disable_p2p_for_sharing = false;
 
+  // True if the Omaha rule instructs us to powerwash.
+  bool powerwash_required = false;
+
   // If not blank, a base-64 encoded representation of the PEM-encoded
   // public key in the response.
   std::string public_key_rsa;