metrics: Switch uploader to use http_utils from libchromeos

Removed explicit dependency on libcurl from metrics and using
chromeos::http::PostText() function instead.

BUG=chromium:411076
TEST=FEATURES=test emerge-link metrics

Change-Id: Ida883fa18d266e9bb87c3a4271e57d44c9308c79
Reviewed-on: https://chromium-review.googlesource.com/216526


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>