Add PowerSaveBlocker in ContentVideoView

This patch add additonal PowerSaveBlocker in ContentVideoView, and will block
screen sleep when ContentView's container isn't visiable.

Upstream change: https://codereview.chromium.org/141553010/

BUG: 12597703

Change-Id: Ic0f64d4efeabac27aa1dbc74decce878343733b9
3 files changed