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Introduces the metadata required to link together the desktop tree and a views::WebView. Additionally, implements the js side logic to resolve these links.

A views::WebView now gets assigned an AXRole of webView and its immediate child resolves to the rootWebArea it hosts. This resolution is done dynamically at runtime using the underlying AX tree id which itself is mapped using AXTreeIDRegistry back in the browser. If the hosting webView has yet to load its child rootWebArea, enableFrame will be called to and a callback waiting for the subroot's data; this auto load occurs as soon as a webView is seen during a tree update and results in a childrenChanged event once its child rootWebArea is available. When a caller asks for the parent of a rootWebArea, a dynamic lookup is also performed to retrieve a hosting webView, if any.

Review URL: https://codereview.chromium.org/667713006

Cr-Commit-Position: refs/heads/master@{#302648}