Fix flaky ShelfAppBrowserTest.MultiDisplayBasicDragAndDrop

Use DisplayManagerTestApi::UpdateDisplay to config multi-display.
(do not use the 'ash-host-window-bounds' command line flag).

TODO: Figure out why this change fixes the flake ¯\_(ツ)_/¯

BUG=678622
TEST=Automated test passes consistently.
R=sky@chromium.org

Review-Url: https://codereview.chromium.org/2628433002
Cr-Commit-Position: refs/heads/master@{#442403}
1 file changed