Support a build flag to optimize binary size

Introduce build flag `enable_perfetto_system_consumer` which allows
excluding only the consumer portion of the SDK's system backend..
When this flag is set to false, the binary size is reduced by ~300 KB.

Chrome on Android isn't allowed to start system tracing sessions, so
there's no point including consumer IPC code in the Chrome binary.
As a result, we don't need consumer_ipc in system backend. Hence, in
those cases, we could save the binary size by not including the
consumer_ipc.

Change-Id: Iccc737c66e9d97f80b2d75f9e6933fec32c807ff

gn/perfetto_integrationtests.gni

diff --git a/gn/perfetto_integrationtests.gni b/gn/perfetto_integrationtests.gni
index 9657a3d..2001d34 100644
--- a/gn/perfetto_integrationtests.gni
+++ b/gn/perfetto_integrationtests.gni
@@ -19,7 +19,7 @@
   "src/tracing/test:client_api_integrationtests",
 ]
 
-if (enable_perfetto_ipc) {
+if (enable_perfetto_ipc && enable_perfetto_system_consumer) {
   perfetto_integrationtests_targets +=
       [ "src/tracing/test:tracing_integration_test" ]
 }