Make base::Environment::Create() return unique_ptrs.

BUG=581865
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win10_chromium_x64_rel_ng
TBR=jshin@chromium.org

Review-Url: https://codereview.chromium.org/2094913002
Cr-Commit-Position: refs/heads/master@{#403115}
14 files changed