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For platforms that support multiple workspaces (currently Mac and Linux), this
allows app windows be visible on all workspaces simultaneously.

API proposal:
https://docs.google.com/document/d/1RC3CYwsrVxS_5hXg6nE3zA9y59G98z9Ezmmmq_Gzx9o/edit?usp=sharing

BUG=384644

Review URL: https://codereview.chromium.org/469993003

Cr-Commit-Position: refs/heads/master@{#293676}