This looks like this is occurring due to the inconsistency between the primary toolbar's max/min heights and the value returned by BVC's |-headerHeight|.  Should be fixable by adjusting BVC's |-updateContentViewPaddingForFullscreenProgress:|

crrev.com/c/1023060

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8f68ae500e9ceda56919b3aa2b1e566b1cd0783d

commit 8f68ae500e9ceda56919b3aa2b1e566b1cd0783d
Author: Kurt Horimoto <kkhorimoto@chromium.org>
Date: Sat Apr 21 18:41:29 2018

[iOS] Scale the top inset based on the toolbar height delta.

Since a fullscreen progress of 0.0 means that kToolbarHeightFullscreen
of the toolbar will still be visible in the UI refresh, the web top
content inset needs to be scaled by the delta between the max and min
heights rather than scaling from |-toolbarHeight| to 0.0

Bug:  833950 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic56f0bc7dc137ecaa5c6ab298156ebc0559ee00c
Reviewed-on: https://chromium-review.googlesource.com/1023060
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552591}
[modify] https://crrev.com/8f68ae500e9ceda56919b3aa2b1e566b1cd0783d/ios/chrome/browser/ui/browser_view_controller.mm

Issue verified 
Version: Chrome Canary  68.0.3405.0
Device: iPhone 6
iOS: 11.2.6

Page header not underneath the toolbar.
https://drive.google.com/open?id=1R9L_A_rWFjLIWQn6LMfAP7VKRZYUhzUL

Note that all the status bar is turning grey while scrolling up. Since this is a different issue from the one reported I will be opening a new one for that specific behavior and closing as verified this one. 

The NextAction date has arrived: 2018-04-25