Added chrome.system.powerSource doc template

Bug: 832054
Change-Id: Ib242e10bc1aeba93ae7850f1127432beb2777ca4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768774Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Tove Petersson <tovep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691526}

chrome/common/extensions/docs/templates/public/apps/system_powerSource.html

+{{+partials.standard_apps_api api:apis.apps.system_power_source/}}