Adds documentation for WindowTreeClient::OnWindowFocused
Adds documentation for WindowTreeClient::OnWindowFocused

BUG=none
TEST=none
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/1863673002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385285}

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