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This CL adds a blur effect to the toolbar's background, to see the
content of the page below.

Screenshot: https://drive.google.com/file/d/1MB5lyrozQ3OkEenWtXi3MWfOy0e2hwxx/view

Bug: 805846
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib12dd0396fbfef9868d352e92a9ec0981aa091df
Reviewed-on: https://chromium-review.googlesource.com/897323
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Louis Romero <lpromero@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533645}