[Layout API] Convert WebViewImpl to use new layout API
[Layout API] Convert WebViewImpl to use new layout API

There is only one reference to LocalFrame::contentLayoutObject(),
in WebViewImpl::contentsSize(). I converted it to use contentLayoutItem()
instead. The method it references is 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/1851813002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384641}

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