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Currently, DirectoryBackingStore::VerifyReferenceIntegrity only verifies
that all parent ids are valid. This ignores the case of a single, corrupted
root node, which can happen during first sync. This CL adds a check that the
root node exists with the expected id("r").

R=pavely@chromium.org

BUG=642690

Review-Url: https://codereview.chromium.org/2342133002
Cr-Commit-Position: refs/heads/master@{#419040}