Add support for a custom test_runner.py script.

build/config/android/internal_rules.gni

Index: build/config/android/internal_rules.gni
diff --git a/build/config/android/internal_rules.gni b/build/config/android/internal_rules.gni
index ee151fbf81360c9175970e7b05ee159ffe40a1bc..3121ee986856da5a63143c0d71770aa16eabe74c 100644
--- a/build/config/android/internal_rules.gni
+++ b/build/config/android/internal_rules.gni
@@ -591,6 +591,13 @@ template("test_runner_script") {
       "--script-output-path",
       rebase_path(generated_script, root_build_dir),
     ]
+    if (defined(android_test_runner_script)) {
+      args += [
+        "--test-runner-path",
+        android_test_runner_script,
+      ]
+    }
+
     args += test_runner_args
   }
 }