Implements N:M forwarding of OmniboxEditModel events on iOS.
RLZTrackerDelegate implementation on iOS need to listen to events from
any OmniboxEditModel (all other clients of the same events on iOS also
do not care about which OmniboxEditModel sends them).
Introduce OmniboxEditModelObserverRelay to allow N:M forwarding. This
class will be informed of events sent by all OmniboxEditModel and will
forward them to all interested observers.
BUG=511570,371536
Review URL: https://codereview.chromium.org/
1245683002
Cr-Commit-Position: refs/heads/master@{#339640}