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Cause:
This is a regression of the HRD change, in which we pass in an unique
ID everytime when we create an app window.  This causes a new window
to be created whenever the user clicks on the app icon in the app
launcher.

Fix:
Only create a new app window when:
1. There are no existing windows or
2. The user clicks on the New Window context menu

BUG=448128

Review URL: https://codereview.chromium.org/844503007

Cr-Commit-Position: refs/heads/master@{#311545}