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/update_engine.xml b/update_engine.xml
index 44b7a7c..ad6c36b 100644
--- a/update_engine.xml
+++ b/update_engine.xml
@@ -13,6 +13,16 @@
       <arg type="s" name="app_version" />
       <arg type="s" name="omaha_url" />
     </method>
+    <!-- TODO(zeuthen,chromium:286399): Rename to AttemptUpdate and
+         update Chrome and other users of the AttemptUpdate() method
+         in lockstep.
+    -->
+    <method name="AttemptUpdateWithFlags">
+      <arg type="s" name="app_version" />
+      <arg type="s" name="omaha_url" />
+      <!-- See AttemptUpdateFlags enum in dbus_constants.h. -->
+      <arg type="i" name="flags" />
+    </method>
     <method name="AttemptRollback">
       <arg type="b" name="powerwash" />
     </method>