Chrome Version: 60.0.3081.6 (Official Build) canary (64-bit)
OS: OSX 10.12.14

What steps will reproduce the problem?
(1) Open a page that scrolls, e.g. https://en.wikipedia.org/wiki/Main_Page
(2) If the scrollbars are visible, wait until they fade out. (If scrollbars remain visible, change setting in System Preferences > General > Show scroll bars > When scrolling.)
(3) Resize the window.

What is the expected result?
Scrollbars should appear during the resize.

What happens instead?
Scrollbars remain hidden.

Note: Safari gets this right, but only for the main frame.  Ideally iframes and scrolling divs should get the same behavior.

I think this just requires plumbing FrameView::ViewportSizeChanged and PLSA::VisibleSizeChanged into ScrollAnimatorMac::SendContentAreaScrolledSoon.