arc-bridge: Enable ARC with a command-line flag

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index ca1c73ac21f42e6fa7f7cfd1972cdb8d40d459cc..26bdc6c564ae080f0cdfeabbd1b6f395586f2681 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -1558,9 +1558,6 @@
     'libjpeg_ijg_gyp_path': '<(DEPTH)/third_party/libjpeg/libjpeg.gyp',
     'libjpeg_turbo_gyp_path': '<(DEPTH)/third_party/libjpeg_turbo/libjpeg.gyp',
 
-    # ARC instance disabled by default.
-    'enable_arc%': 0,
-
     'conditions': [
       ['buildtype=="Official"', {
         # Continue to embed build meta data in Official builds, basically the
@@ -3102,9 +3099,6 @@
         # TODO: Enable on Windows too, http://crbug.com/404525
         'variables': { 'clang_warning_flags': ['-Wexit-time-destructors']},
       }],
-      ['<(enable_arc)==1', {
-        'defines': ['ENABLE_ARC=1'],
-      }],
       ['chromium_code==0', {
         'variables': {
           'clang_warning_flags': [