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FrameSinkElement creation is moved into a function that creates the
unique_ptr and adds it to the map of elements. Removing elements from
this map now deletes them.

The FrameSinkElements need to be owned by DOMAgentViz because it
receives all of the FrameSinkObserver events. Whereas for Views, each
ViewElement, WidgetElement, and WindowElement is its own Observer, so
centralized ownership in DOMAgentAura doesn't make as much sense.

Elements are no longer inserted into |frame_sink_elements_| by
BuildTreeForFrameSink(), so we don't need to static_cast pointers any
more. Also make variable names in BuildTreeForFrameSink() clearer.

Bug: 816802
Change-Id: Ieec4db482ffb3ff8a740eb559c1c3d9ee9a80705
Reviewed-on: https://chromium-review.googlesource.com/c/1346910
Commit-Queue: Sean Gilhuly <sgilhuly@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613202}