If 'vertical-scroll' is disabled for an <iframe>, then it should not be
able to affect the vertical scroll position. One way to block is to use
scripted scrolling by calling "element.scrollIntoView()".

To block such frames (whose feature's disabled), programmatic recursive
scroll calls are not forwarded to parent frames. This means if a given
<iframe> is blocked, then all the calls to scrollIntoView() are limited
to the scope of frame (i.e., elements becoming visible in the frame).
This applies to all the nested <iframe>'s of a disabled frame as well
since they would have the feature disabled as part of propagating the
container policy.

Link to explainer/design document for "vertical-scroll":
https://docs.google.com/document/d/1qiWelnMlsOHuT_CQ0Zm_qEAf54HS5DhoIvEDHBlfqps/edit?usp=sharing

Bug: 611982
Change-Id: I0e06b399ad890e263128b997cfbb04eb3bdd1494
Reviewed-on: https://chromium-review.googlesource.com/1014191
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553561}