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- This CL adds AshDevToolsDOMAgent as an observer to all widgets and views.
The WidgetRemovalObserver is specifically to remove the RootView subtree,
while the ViewObserver is to update any other descendant views.
AddViewTree was created so that whenever any |view| is added, it can
be pushed to the frontend.
- Add tests for when views are inserted, removed, rearranged,
removed & inserted.
- Add test to check if a widget owned by a native widget is represented
properly in the DOM.

BUG=648701

Review-Url: https://codereview.chromium.org/2537163002
Cr-Commit-Position: refs/heads/master@{#435065}