make v8_use_external_startup_data overridable

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index c899e14d4bf06b80ac8bf7f7c478145b59e4caab..8158afe33b01d077e8849fb7d0f3e7cb9e77eda6 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -1001,9 +1001,9 @@
         # TODO(baixo): Enable v8_use_external_startup_data
         # http://crbug.com/421063
         ['android_webview_build==0 and android_webview_telemetry_build==0 and chromecast==0 and chromeos==0 and (OS=="android" or OS=="linux" or OS=="mac")', {
-          'v8_use_external_startup_data': 1,
+          'v8_use_external_startup_data%': 1,
         }, {
-          'v8_use_external_startup_data': 0,
+          'v8_use_external_startup_data%': 0,
         }],
       ],
 
@@ -1207,7 +1207,7 @@
     'video_hole%': '<(video_hole)',
     'enable_load_completion_hacks%': '<(enable_load_completion_hacks)',
     'support_pre_M6_history_database%': '<(support_pre_M6_history_database)',
-    'v8_use_external_startup_data': '<(v8_use_external_startup_data)',
+    'v8_use_external_startup_data%': '<(v8_use_external_startup_data)',
 
     # Whether or not we are building the Athena shell.
     'use_athena%': '0',