libchromeos: Replace scoped_ptr with std::unique_ptr in async http callbacks

Changed callbacks to use std::unique_ptr in async HTTP response callbacks
instead of scoped_ptr, now that we can do this with base::Callback.
This eliminates the need to mix scoped_ptr and std::unique_ptr in the same
translation unit.

Sweep the code in leaderd and buffet to match the signature of the callbacks.

BUG=None
TEST=`FEATURES=test emerge-link libchromeos leaderd buffet`
CQ-DEPEND=CL:*214526

Change-Id: I2f93db3f863b2fb1994058b2b1fff12760edd022
Reviewed-on: https://chromium-review.googlesource.com/267342
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
10 files changed