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Based on the comment from:
https://codereview.chromium.org/2155743002/

This CL changes ChooserController::Observer to ChooserController::View
to better reflect its purpose, since ChooserController needs to update
its view when necessary.

BUG=629878

Review-Url: https://codereview.chromium.org/2163683005
Cr-Commit-Position: refs/heads/master@{#406696}