Use inert-visual-viewport flag correctly.

With the inert-visual-viewport flag set, window.scrollBy and window.scrollTo
should only scroll the layout viewport. Found a logic error where we don't
check this flag before calculating current offset.

This CL also updates the visual viewport to only fire scroll event when the
inert-visual-viewport flag is off.

BUG=489206

Review URL: https://codereview.chromium.org/1419143007

Cr-Commit-Position: refs/heads/master@{#357887}
2 files changed