Renamed build output in brave/network/public/cpp.

Due to a collision with another target:

ERROR at //build/config/BUILDCONFIG.gn:570:5: Duplicate output file.
    target(_target_type, _target_name) {
    ^-----------------------------------
Two or more targets generate the same output:
  cpp.dll

This is can often be fixed by changing one of the target names, or by
setting an output_name on one of them.

Collisions:
  //brave/services/network/public/cpp:cpp
  //media/learning/mojo/public/cpp:cpp

Chromium change:

https://chromium.googlesource.com/chromium/src/+/96665a836d2b2ffab4951167fbf3230dbe3aec30

commit 96665a836d2b2ffab4951167fbf3230dbe3aec30
Author: liberato@chromium.org <liberato@chromium.org>
Date:   Thu Jan 23 00:35:37 2020 +0000

    Attach SmoothnessHelper to WMPI for MediaCap experiment

    This CL also plumbs ukm::SourceId through BeginObservation, instead
    of CompleteObservation so that default target values work.

services/network/public/cpp/BUILD.gn

@@ -6,6 +6,8 @@ jumbo_component("cpp") {
     "//services/network/public/cpp:cpp",
   ]
 
+  output_name = "brave_network_cpp"
+
   sources = [
     "system_request_handler.cc",
     "system_request_handler.h",