This method was added to let mash set the widget container back when
mash could not use aura::Window. Now we can set the parent aura::Window
explicitly.

Eliminate the TrayBubbleView::Create() factory method because its
signature is identical to the constructor and it doesn't do any
validation.

Eliminate some GetInitParams / GetDefaultInitParams methods that were
only used by WebNotificationTray.

This is mostly a manual revert of:
https://codereview.chromium.org/2092473002/

BUG=671246
TEST=ash_unittests, views_unittests, manually opening all system tray
bubbles on primary and secondary monitors

Review-Url: https://codereview.chromium.org/2901273003
Cr-Commit-Position: refs/heads/master@{#475682}