Don't make two compositors in app list unittest
Don't make two compositors in app list unittest

On non-ChromeOS platforms, this code would cause two ui::Compositors to
be created, one for the AuraTestHelper and the other for the
BubbleWidget.

Clean this code up so that the BubbleWidget view uses the AuraTestHelper
root window and compositor when Aura is turned on.

Review URL: https://codereview.chromium.org/1153093005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#333354}

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