Fix unique_ptr usage in handling OAuth2TokenService::StartRequest result

Calling release() on a temporary unique_ptr to reset another unique_ptr with
resulting raw pointer is verbose and obscure. It also loses custom deleter
(see C++11 standard, clause 20.7.1.2.3.1).

BUG=none

Review-Url: https://codereview.chromium.org/2755403002
Cr-Original-Commit-Position: refs/heads/master@{#458047}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e32318796eaf782ee8afe8ec047b1288f756f644
1 file changed