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This CL extracts the logic of querying battery state from
PowerDrainRecorder into a new BatteryLevelProvider, to eventually
reuse on other platforms.
BatteryLevelProvider is platform specific. This CL implements mac only.
Other platforms will be implemented in follow-up CLs.

Bug: 1153193
Change-Id: I56bca3b851428483395e5edf6279087cd293e7e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574438
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840826}