GitLab

Previously AuraWindowRegistry was allocating ID 0 to first window. This
breaks assumptions made in other places, e.g. DCHECK in
DesktopMediaPickerDialogView. After this change the implementation
guarantees that window IDs are always unique positive integers.

Review URL: https://codereview.chromium.org/499813002

Cr-Commit-Position: refs/heads/master@{#291708}