Visual viewport API initial implementation.

Adds a new visualViewport object to document that contains visual
viewport information.

A visualviewportchanged event fires everytime the visual viewport is
resized or scrolled.

WICG proposal: https://github.com/WICG/ViewportAPI

BUG=595826

Review URL: https://codereview.chromium.org/1814013002

Cr-Commit-Position: refs/heads/master@{#385047}
16 files changed