Make FakePowerManagerClient::policy() return a const ref.
Make FakePowerManagerClient::policy() return a const ref.

It looks like I accidentally made this accessor return a
non-const reference long ago.

BUG=none

Review URL: https://codereview.chromium.org/1412743002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354566}

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