Seperate DOMVisualViewport from Blink's VisualViewport class
DOMVisualViewport represents the things exposed via the VisualViewportAPI. This
is what's returned from window.visualViewport. It queries the Blink's
VisualViewport for the main frame and returns sane values for iframes.
BUG=
604928
Committed:
https://crrev.com/81dd83a5bae93da969e909c58ed3ecb50d50100c
Cr-Commit-Position: refs/heads/master@{#401616}