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Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

I also still need to create some tests.

The tests will fail until the engine changes roll into framework, but here are the golden image results I'm expecting...

All implementations of the back end for this widget have landed in the Engine repo, and we're just waiting for those commits to roll to the framework repo to finalize this PR. Here is a screenshot of the new ImageFiltered running on Chrome (note that the BackdropFilter doesn't produce a blur yet on Chrome, it simply dims the background, so the ImageFiltered widget will become a preferred way to blur a background if you can use it in your app).

LGTM from the engine interaction standpoint. I left some comments on the API design, but consider them non-blocking. API LGTM if LGT @Hixie.

The engine implementation for C++ has rolled into the framework repo with web stubs, and the web implementation that removes the stubs is staged in the engine roller (just waiting for the tree to go green to be rolled), so I am removing the WIP label.

This PR should be ready to go.

modulo comments