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Combine ManifestHandlers for PageActions and BrowserActions.
Remove support for the uber-deprecated "page_actions" (plural) manifest key.
Clean up related tests.

TBR=avi@chromium.org (c/b/ui/cocoa)
BUG=419475

Review URL: https://codereview.chromium.org/624453003

Cr-Commit-Position: refs/heads/master@{#297927}