[gn] Switch off gyp in runhooks by default

This will be synced with an announcement on v8-dev. This
doesn't affect directly calling gyp in our old Makefiles.

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2327033002
Cr-Commit-Position: refs/heads/master@{#39308}

DEPS

@@ -279,6 +279,6 @@ hooks = [
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
-    "action": ["python", "v8/gypfiles/gyp_v8"],
+    "action": ["python", "v8/gypfiles/gyp_v8", "--running-as-hook"],
   },
 ]

gypfiles/gyp_v8

@@ -118,10 +118,19 @@ def run_gyp(args):
 if __name__ == '__main__':
   args = sys.argv[1:]
 
-  if int(os.environ.get('GYP_CHROMIUM_NO_ACTION', 0)):
+  gyp_chromium_no_action = os.environ.get('GYP_CHROMIUM_NO_ACTION')
+  if gyp_chromium_no_action == '1':
     print 'Skipping gyp_v8 due to GYP_CHROMIUM_NO_ACTION env var.'
     sys.exit(0)
 
+  running_as_hook = '--running-as-hook'
+  if running_as_hook in args and gyp_chromium_no_action != '0':
+    print 'GYP is now disabled by default in runhooks.\n'
+    print 'If you really want to run this, either run '
+    print '`python gypfiles/gyp_v8` explicitly by hand '
+    print 'or set the environment variable GYP_CHROMIUM_NO_ACTION=0.'
+    sys.exit(0)
+
   gyp_environment.set_environment()
 
   # This could give false positives since it doesn't actually do real option