Allow extension APIs to be compiled out.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index d56a3b5d02a6c87a6c0e00b7ec696a53d2127aa1..9e52cc6c7dddb587a5ff45380c700b0c2f1f9310 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -218,6 +218,9 @@
       # and extensions Web Intents support.
       'enable_web_intents%': 1,
 
+      # Enable Chrome browser extensions
+      'enable_extensions%': 1,
+
       # Enable Web Intents web content registration via HTML element
       # and WebUI managing such registrations.
       'enable_web_intents_tag%': 0,
@@ -503,6 +506,7 @@
     'asan%': '<(asan)',
     'order_text_section%': '<(order_text_section)',
     'enable_register_protocol_handler%': '<(enable_register_protocol_handler)',
+    'enable_extensions%': '<(enable_extensions)',
     'enable_web_intents%': '<(enable_web_intents)',
     'enable_web_intents_tag%': '<(enable_web_intents_tag)',
     'enable_plugin_installation%': '<(enable_plugin_installation)',
@@ -795,6 +799,7 @@
         'configuration_policy%': 0,
         'input_speech%': 0,
         'enable_web_intents%': 0,
+        'enable_extensions%': 0,
         'java_bridge%': 1,
         # Android does not support themes.
         'enable_themes%': 0,
@@ -1370,6 +1375,11 @@
           'ENABLE_WEB_INTENTS=1',
         ],
       }],
+      ['enable_extensions==1', {
+        'defines': [
+          'ENABLE_EXTENSIONS=1',
+        ],
+      }],
       ['OS=="win" and branding=="Chrome"', {
         'defines': ['ENABLE_SWIFTSHADER'],
       }],