v8gen.py: change file open mode from 'aw' to just 'a'

'a' and 'w' are mutually exclusive and just 'a' is what we actually
want. Linux doesn't care (it just takes the first one) but Windows
throws a ValueError.

R=machenbach@chromium.org
BUG=None

Review-Url: https://codereview.chromium.org/2467363002
Cr-Commit-Position: refs/heads/master@{#40711}

tools/dev/v8gen.py

@@ -269,7 +269,7 @@ class GenerateGnArgs(object):
     # Artificially increment modification time as our modifications happen too
     # fast. This makes sure that gn is properly rebuilding the ninja files.
     mtime = os.path.getmtime(gn_args_path) + 1
-    with open(gn_args_path, 'aw'):
+    with open(gn_args_path, 'a'):
       os.utime(gn_args_path, (mtime, mtime))
 
     return True