Fire visualviewportchanged event on window rather than document

In https://codereview.chromium.org/2025393003/ I moved the visualViewport object
from document onto window but forgot to move the event to fire on the window
instead.

BUG=595826

Review-Url: https://codereview.chromium.org/2034313002
Cr-Commit-Position: refs/heads/master@{#398067}
3 files changed