Move document.visualViewport to window.visualViewport

The `window` object is a more natural place for the visualViewport API. The
original concern was causing breaking changes since this would clash with any
variable in the global scrope called `visualViewport` but a search through
httpArchive found that this is unlikely to be an issue.


Cr-Commit-Position: refs/heads/master@{#397845}
21 files changed