PepperPortAllocator and ChromiumPortAllocator were different only in
how they were sending HTTP requests to allocate relay session. They were
using pp::URLLoader and net::URLFetcher respectively, but otherwise
contained largely the same logic. Now URL request logic is abstracted
with remoting::UrlRequest interface and so a single PortAllocator
implementation can be used in all cases.

BUG=577954

Review URL: https://codereview.chromium.org/1681393006

Cr-Commit-Position: refs/heads/master@{#375042}