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Currently, these are created with bare `new`, until they are threaded
into the ContentSettingBubbleContents, where they are wrapped into a
unique_ptr.

This CL removes the bare new creation, so ownership passing is explicit.

Bug: None
Change-Id: Ieaecab0ecaa49ae0d732acb27615b6b2e328e167
Reviewed-on: https://chromium-review.googlesource.com/c/1333871Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607691}