Enable the use of system python in testing.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 300bb4761ebd1f2c9bce01a3bf8e5b8bc472cc3a..3d5d25dd175952161055507fd05f4bdab72340f9 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -233,6 +233,9 @@
       # use_libjpeg_turbo is set.
       'use_system_libjpeg%': 0,
 
+      # Use system python instead of the one in third_party.
+      'use_system_python_in_tests%': 0,
+
       # By default, component is set to static_library and it can be overriden
       # by the GYP command line or by ~/.gyp/include.gypi.
       'component%': 'static_library',
@@ -806,6 +809,7 @@
     'wix_path%': '<(wix_path)',
     'use_libjpeg_turbo%': '<(use_libjpeg_turbo)',
     'use_system_libjpeg%': '<(use_system_libjpeg)',
+    'use_system_python_in_tests%': '<(use_system_python_in_tests)',
     'android_webview_build%': '<(android_webview_build)',
     'gyp_managed_install%': 0,
     'google_tv%': '<(google_tv)',
@@ -2138,6 +2142,11 @@
           }],
         ],  # win_z7!=0
       }],  # OS==win
+      ['use_system_python_in_tests==1', {
+        'defines': [
+          'USE_SYSTEM_PYTHON_IN_TESTS=1',
+        ],
+      }],
       ['enable_task_manager==1', {
         'defines': [
           'ENABLE_TASK_MANAGER=1',