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This is primarily to verify behavior fixed in https://codereview.chromium.org/2616633002/ but also adds tests for more fundamental behavior of the BookmarkSuggestionsProvider.

This CL also moves ContentSuggestionsProvider test doubles out of specific unit tests and into separate targets so that we can reuse them.

BUG=674178

Review-Url: https://codereview.chromium.org/2618243004
Cr-Commit-Position: refs/heads/master@{#443260}