Remove inert-visual-viewport flag (enabled by default)

This patch removes the --enable-inert-visual-viewport flag from Chrome.
This does *not* remove the inertVisualViewport setting from Blink. This
feature is currently on by default but it was accidentally forced to the
off state by the existence of the command line flag. By removing the
flag, the Blink setting controls the status of this switch - which is
currently set to on.

Note: we're not losing anything as the feature is intended to be on by
default and we didn't have a matching --disable-inert-visual-viewport
flag so this flag would have been useless as of M61.

This patch fixes some tests that were expecting the flag to be off and
removes others that were explicitly testing behavior of the flag when
disabled. I also added a new test to test the new flag-on behavior.

This also exposed a bug in document.rootScroller, window's scrollX
and scrollY properties reverted to returning the FrameView's rather
than the rootScroller's scroll offset. This was caught by the
set-root-scroller.html test, I've commented out the checks
(rootScroller isn't yet shipped), and I'll fix the behavior in a
followup patch.

Bug: 740956
Change-Id: I249a34b4ee67dfbe8230e6bc3048871a6325e43d
Reviewed-on: https://chromium-review.googlesource.com/575768
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488018}
20 files changed