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1. https://chromium-review.googlesource.com/2171436 introduced a change
   to how `AXPlatformNodeBase::IsLeaf` works; previously, a node would
   start by querying `AXPlatformNodeBase::GetChildCount` (which relies
   on `BrowserAccessibility::PlatformChildCount`) to check whether the
   node has no children (which would make it a leaf by definition).

   After CL 2171436, `AXPlatformNodeBase::IsLeaf` is delegated to
   `BrowserAccessibility::PlatformIsLeaf`, but the case of an '<iframe>'
   is mishandled since now we don't check if the node has a child tree.

   Fixing `BrowserAccessibility::IsLeaf` to check for the child tree of
   a node; this will allow several `BrowserAccessibilityPosition`'s
   methods to correctly traverse into '<iframe>' elements.

2. Introducing a content browser test to validate that methods from
   `BrowserAccessibilityPosition` can navigate into iframes.

3. Rebaselining `DumpAccessibilityTreeTest.AccessibilityIframe` which
   incorrectly expected '<iframe>' nodes to be treated as leaves.

Bug: 1101480
Change-Id: Ib364ee78a39ef6b7854fc41985bd059dfced8a46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2364049Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Ethan Jimenez <ethavar@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#800321}