[Layout API] Convert InspectorOverlay to use new LocalFrame::contentLayoutItem()
[Layout API] Convert InspectorOverlay to use new LocalFrame::contentLayoutItem()

There are several references to LocalFrame::contentLayoutObject(), which
is deprecated by the new layout API. I converted all of them to use
contentLayoutItem() instead. The methods referenced are already part of
the layout API, so no new API methods are required.

There are no functional changes.

BUG=499321

Review URL: https://codereview.chromium.org/1865053002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385944}

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