[Android] Fix gn build for instrumentation tests.

tools/android/forwarder2/forwarder.gyp

Index: tools/android/forwarder2/forwarder.gyp
diff --git a/tools/android/forwarder2/forwarder.gyp b/tools/android/forwarder2/forwarder.gyp
index c4273660c57cfafded1a4a6e6a3fd5b10989f9d6..54bf8b723880d90fce695190df5625ef986f1a45 100644
--- a/tools/android/forwarder2/forwarder.gyp
+++ b/tools/android/forwarder2/forwarder.gyp
@@ -3,8 +3,12 @@
 # found in the LICENSE file.
 
 {
+  'variables': {
+    'chromium_code': 1,
+  },
   'targets': [
     {
+      # GN version: //tools/android/forwarder2
       'target_name': 'forwarder2',
       'type': 'none',
       'dependencies': [
@@ -20,6 +24,7 @@
       'includes': ['../../../build/android/native_app_dependencies.gypi'],
     },
     {
+      # GN version: //tools/android/forwarder2:device_forwarder
       'target_name': 'device_forwarder',
       'type': 'executable',
       'toolsets': ['target'],
@@ -45,6 +50,7 @@
       ],
     },
     {
+      # GN version: //tools/android/forwarder2:host_forwarder
       'target_name': 'host_forwarder',
       'type': 'executable',
       'toolsets': ['host'],