Set flags for confirm dialog buttons

"Discard edits" isn't constructive, and "Continue editing" isn't destructive.
Instead, mark "Discard edits" as destructive, and make the continue
button neutral (no flags).

Change-Id: I7648555ad47be698e75b5019d7738b0afb8611aa
(cherry picked from commit 5b93980b68cbe8862b400696e979ec93a59d2b8f)
Merge "Set flags for confirm dialog buttons" into wmf/1.24wmf5