Bug 1586986 - Fire visual viewport resize events and flush position:fixed elements' layout in the same way what Chrome does. r=botond

On Chrome, visual viewport resize event is fired repeatedly during dynamic
toolbar transitions and visual viewport height obtained by the VisualViewport
API is also changed, but in terms of layout the height value is never used
until the dynamic toolbar height reaches to zero or is changed from zero.
The height used at the time is the height for vh units when the toolbar height
reaches to zero and the ICB height when the toolbar height is changed from zero.
To do so, we need to have another visual viewport size in parallel to the
original one and use them depending on situations.

Differential Revision: https://phabricator.services.mozilla.com/D52338

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extra : moz-landing-system : lando