Don't build interceptor_trace_writer_unittest.cc in chromium

Due to already known build complications explained in an adjacent TODO,
the test cannot be build in chromium (other neighboring tests have a
similar problem).

Change-Id: I8da9d0c1b0b0f374a8ab65f03921ac95b35607fc
diff --git a/src/tracing/BUILD.gn b/src/tracing/BUILD.gn
index 5635c56..db37d8c 100644
--- a/src/tracing/BUILD.gn
+++ b/src/tracing/BUILD.gn
@@ -147,7 +147,7 @@
     "test:test_support",
   ]
 
-  sources = [ "internal/interceptor_trace_writer_unittest.cc" ]
+  sources = []
 
   # TODO(primiano): remove the build_with_chromium conditional once the root
   # //BUILD.gn:libperfetto (in chromium) stops adding tracing:platform_fake.
@@ -163,6 +163,7 @@
     ]
 
     sources += [
+      "internal/interceptor_trace_writer_unittest.cc",
       "traced_proto_unittest.cc",
       "traced_value_unittest.cc",
     ]