Remove ScopedVector usage from components/cronet

This CL replaces ScopedVector with std::vector<std::unique_ptr>
in components/cronet.
ScopedVector is deprecated, see bug.

BUG=554289

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: I93ed1cac20093344cc9d46c648f02cb579629794
Reviewed-on: https://chromium-review.googlesource.com/536240


Commit-Queue: Han Leon <leon.han@intel.com>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481036}