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_request_action.cc b/omaha_request_action.cc
index 1becb8e..6354641 100644
--- a/omaha_request_action.cc
+++ b/omaha_request_action.cc
@@ -76,6 +76,7 @@
 static const char* kTagDisableP2PForDownloading = "DisableP2PForDownloading";
 static const char* kTagDisableP2PForSharing = "DisableP2PForSharing";
 static const char* kTagPublicKeyRsa = "PublicKeyRsa";
+static const char* kTagPowerwash = "Powerwash";
 
 static const char* kOmahaUpdaterVersion = "0.1.0.0";
 
@@ -1085,6 +1086,7 @@
 
   output_object->disable_payload_backoff =
       ParseBool(attrs[kTagDisablePayloadBackoff]);
+  output_object->powerwash_required = ParseBool(attrs[kTagPowerwash]);
 
   return true;
 }