Enable HighEfficiencyMode by default: activation of auto freeze in background...

Enable HighEfficiencyMode by default: activation of auto freeze in background tabs with high CPU consumption.

Only valid for desktop platform: activation of auto freeze in
background tabs with high CPU consumption, which is not visible from
the ui and therefore not user controllable.
The feature is controlled by the "freezing-without-battery-saver" flag
active by default.