Convert //content from scoped_ptr to std::unique_ptr

BUG=554298
R=avi@chromium.org
TBR=rdevlin.cronin@chromium.org,thestig@chromium.org

Review URL: https://codereview.chromium.org/1874903002

Cr-Commit-Position: refs/heads/master@{#386303}
214 files changed