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Make the Battery Status API available only within a secure context that is
also a top-level browsing context. This disallows the use of the API within
framed content, as well as from any content that is not a secure context.

Details: https://github.com/w3c/battery/issues/10

WPT updated in https://github.com/w3c/web-platform-tests/pull/5871

BUG=661792

Review-Url: https://codereview.chromium.org/2880763002
Cr-Commit-Position: refs/heads/master@{#476263}