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This CL is to confirm that in a real browser environment javascript
battery APIs(navigator.getBattery and its event listener) CAN eventually
succeed in connecting to Device Service to receive battery status change
notifications correctly.

BUG=717379
TEST=content_browsertests

Review-Url: https://codereview.chromium.org/2882633002
Cr-Commit-Position: refs/heads/master@{#473432}