update_engine_client: Add -interactive=true|false flag

This makes it easier to test the update_engine codepaths dealing with
non-interactive update attempts.

BUG=chromium:273251
TEST=Manually tested with 'update_engine_client -update -interactive=X' +
    unit tests pass.

Change-Id: Id3894261fd1c0dc2afdf2426484835f487924433
Reviewed-on: https://chromium-review.googlesource.com/168445
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index a89bcac..dee64ee 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -18,6 +18,9 @@
            send_member="AttemptUpdate"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
+           send_member="AttemptUpdateWithFlags"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
            send_member="AttemptRollback"/>
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"