Hmm, without a meta viewport I would expect us to use the fallback width of 980px, regardless of content.

Even if the content is smaller than 800x600?  Below is a sample layer tree dump for a page that only has a 200x200 div.  It's weird that the scrolling contents layer is 980x735 but the main graphics layer is 800x600.

{
  "layers": [
    {
      "name": "LayoutView #document",
      "bounds": [800, 600],
      "backgroundColor": "#D3D3D3"
    },
    {
      "name": "Scrolling Layer",
      "bounds": [800, 600],
      "drawsContent": false
    },
    {
      "name": "Scrolling Contents Layer",
      "bounds": [980, 735],
      "contentsOpaque": true,
      "backgroundColor": "#D3D3D3"
    },
    {
      "name": "LayoutBlockFlow HTML",
      "bounds": [980, 735],
      "backgroundColor": "#D3D3D3"
    },
    {
      "name": "LayoutBlockFlow (positioned) DIV id='underbody'",
      "bounds": [200, 200],
      "contentsOpaque": true,
      "backgroundColor": "#008000",
      "transform": 1
    },
    {
      "name": "LayoutBlockFlow HTML (foreground) Layer",
      "bounds": [980, 735]
    }
  ],
  "transforms": [
    {
      "id": 1,
      "transform": [
        [1, 0, 0, 0],
        [0, 1, 0, 0],
        [0, 0, 1, 0],
        [8, 8, 0, 1]
      ]
    }
  ]
}

Isn't LocalFrameView::layout_size_ established before the content is laid out?  I think it will have width = 980 on mobile.  As a result, LayoutView::frame_rect_ (ICB) would be 980, and its LayoutOverflowRect() will be 980 as well (if there is no overflow).

But that layer tree looks wrong because I would expect the LayoutView's main GraphicsLayer and Scrolling Layer to also be 980, reflecting the size of the layout viewport.

However, these tests don't seem to be enabling mobile viewport settings.  Do you mean they're only failing on Android bots?

layout_size_ is now set after the page content has been laid out, in LayoutView::UpdateAfterLayout.  If the page content is smaller than 800x600, I don't know why the ICB would be set to 980x735.

These tests are only failing on Android bots.

For context, here is how the layout size gets set:

LayoutView::UpdateAfterLayout
 ChromeClientImpl::ResizeAfterLayout
  WebViewImpl::ResizeAfterLayout
   WebViewImpl::RefreshPageScaleFactorAfterLayout
    WebViewImpl::UpdatePageDefinedViewportConstraints
     WebViewImpl::UpdateMainFrameLayoutSize
      LocalFrameView::UpdateLayoutSize

I haven't succeeded running tests locally yet, due to issue 567947 presumably. However, opening one of the mentioned tests in a local Chrome build, the LayoutView and scrolling layers are also 980, as I'd exepect. So my guess is this works correctly but that we're taking the layer snapshot in the test before we do the "post layout resize". It may just be we have to fix the tests.

I'm still trying to get the tests to run but I might have to reimage my device which will take some time. It may be that we just have to fix the tests to delay the snapshot a bit.

As szager@ pointed out, this is likely related to issue 755405 rather than root-layer-scrolls.