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This makes the app indicator API in libgtkui usable by non-Chrome apps (in
particular, Electron).

Change-Id: I39e547fc04595900d99806208955c632e4199be4
Reviewed-on: https://chromium-review.googlesource.com/c/1315840
Commit-Queue: Jeremy Apthorp <jeremya@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605435}