Allow location of test_runner.py to be overriden in GYP

build/android/test_runner.gypi

Index: build/android/test_runner.gypi
diff --git a/build/android/test_runner.gypi b/build/android/test_runner.gypi
index b211995965fc95511b6389b36744b4b11272dabe..50e3ffa10d0ef5e0f1e39ce0cd7e505fece9b66f 100644
--- a/build/android/test_runner.gypi
+++ b/build/android/test_runner.gypi
@@ -43,6 +43,7 @@
     'variables': {
       'additional_apks%': [],
       'isolate_file%': '',
+      'test_runner_path%': '',
     },
     'test_runner_args': ['--output-directory', '<(PRODUCT_DIR)'],
     'conditions': [
@@ -74,6 +75,9 @@
       ['isolate_file != ""', {
         'test_runner_args': ['--isolate-file-path', '<(isolate_file)']
       }],
+      ['test_runner_path != ""', {
+        'test_runner_args': ['--test-runner-path', '<(test_runner_path)']
+      }],
     ],
   },
   'actions': [