style: Factor dynamic toolbar max height into layout metrics.
style: Factor dynamic toolbar max height into layout metrics.

Now
* nsPresContext::mVisibleArea is excluding the toolbar max height so that
  ICB is now static regardless of the dynamic toolbar transition
* nsPresContext::mSizeForViewportUnits is introduced to resolve viewport units
  which is including the toolbar max height

That means that with the dynamic toolbar max height;

  mVisibleArea < mSizeForViewportUnits

See https://github.com/bokand/URLBarSizing for more detail backgrounds of this
change.

Depends on D50417

Differential Revision: https://phabricator.services.mozilla.com/D50418

splav/servo,splav/servo,splav/servo,DominoTree/servo,KiChjang/servo,DominoTree/servo,KiChjang/servo,DominoTree/servo,splav/servo,splav/servo,DominoTree/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,splav/servo,DominoTree/servo,splav/servo,DominoTree/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo