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There's not really much point in the distinction between page and
browser actions anymore, and we'd like to merge them. Make the
declarativeContent.showPageAction API work with browser actions.
A follow-up will introduce declarativeContent.showAction.

Bug: 893375
Change-Id: Ib4651c225099c62070239f0c37cb4b075d124b1c
Reviewed-on: https://chromium-review.googlesource.com/c/1270006Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611019}