Stop using SpawnedTestServer in DownloadContentTest.*

This CL introduces TestDownloadRequestHandler, a URLRequestJob based
interceptor that can be used to mock URLRequest behavior. The new
request handler supports simulating network and server errors, and is a
replacement for the testserver.py based test setup //content is
currently using to test download resumption.

Moving away from SpawnedTestServer allows us to re-enable the tests on
Android.

R=rdsmith@chromium.org,svaldez@chromium.org,sky@chromium.org
BUG=215894
BUG=493347
BUG=7648

Review URL: https://codereview.chromium.org/1203983004

Cr-Commit-Position: refs/heads/master@{#363108}
7 files changed