The nullptr ProxyService in the context builder triggers a DCHECK on linux and android builds.

This issue was missed in the initial commit due to lack of tests. A unittest should be added to verify solution.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/20f84f3e73b8f4330659fa53ed11b9b5bc642d80

commit 20f84f3e73b8f4330659fa53ed11b9b5bc642d80
Author: jessicag <jessicag@chromium.org>
Date: Mon May 09 18:54:27 2016

Fix DCHECK failure and add unittest for BlimpSystemURLContextGetter.

The URLRequestContextBuilder::Build calll in this function requires at least a dummy ProxyService to be set on linux builds. The nullptr ProxyService field in the context builder triggers a DCHECK on linux and android builds.

Also adding a minimal test to confirm fix and avoid regressions.

BUG= 609998 

Review-Url: https://codereview.chromium.org/1960813002
Cr-Commit-Position: refs/heads/master@{#392384}

[modify] https://crrev.com/20f84f3e73b8f4330659fa53ed11b9b5bc642d80/blimp/engine/BUILD.gn
[modify] https://crrev.com/20f84f3e73b8f4330659fa53ed11b9b5bc642d80/blimp/engine/app/blimp_system_url_request_context_getter.cc
[add] https://crrev.com/20f84f3e73b8f4330659fa53ed11b9b5bc642d80/blimp/engine/app/blimp_system_url_request_context_getter_unittest.cc