[Telemetry] Fix Tab and Extension unittests for CrOS.

My previous fix was incomplete. The problem was that InspectorBackend still
tried to call some things on the TabListBackend. This removes the two bad
dependencies. We should continue removing the dependencies on TabListBackend
from InspectorBackend.

BUG=345875,233001
TEST=$ tools/telemetry/run_tests --browser=cros-chrome Tab Extension

Review URL: https://codereview.chromium.org/180693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253472 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253472
fetch_client needs to call set_proxy_config_service on Android and Linux.
This is similar to the change to url_request_context_builder_unittest.cc
in https://codereview.chromium.org/10155004.

R=eroman@chromium.org,willchan@chromium.org
BUG=342145
TEST=Run a debug build of fetch_client on Linux. It should not hit a
DCHECK failure.

Review URL: https://codereview.chromium.org/180363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253473 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253473