GitLab

This is an AlertOverlayCoordinator subclass that aggregates all the
mediator types used for the supported OverlayRequests for alerts shown
in OverlayModality::kWebContentArea, allowing for the removal of the
boilerplate coordinator code for these overlays.  After this CL, new
alerts shown over the web content area simply need a new mediator that
is added to the coordinator's |supportedMediatorClasses| array.

This CL also creates a new util function to instantiate an appropriate
mediator for other OverlayRequestCoordinators that support multiple
mediator types.  InfobarBannerOverlayCoordinator was updated to use
this function as well, since it also supports multiple banner mediator
classes.

Bug: none
Change-Id: I360b0f94bb2eb76709074f826854a262fc82ce80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149687
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759767}