grit: add support for standard --help flag

Trying to use --help with grit just ends in errors like:
  $ grit --help
  getopt.GetoptError: option --help not recognized

Add support for it with `grit --help`.  This doesn't update the
subtools for now as that'll take much more work/thought.

We also don't support the -h short option as it's already used for
other things, and we don't want to complicate/break it.

Bug: 747171
Change-Id: Ied24f810ed583d464dc2970c8e472784e22ec044
Reviewed-on: https://chromium-review.googlesource.com/c/1327421Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608196}

tools/grit/grit/grit_runner.py

@@ -155,7 +155,7 @@ class Options(object):
 
   def ReadOptions(self, args):
     """Reads options from the start of args and returns the remainder."""
-    (opts, args) = getopt.getopt(args, 'vxi:p:h:')
+    (opts, args) = getopt.getopt(args, 'vxi:p:h:', ('help',))
     for (key, val) in opts:
       if key == '-h': self.hash = val
       elif key == '-i': self.input = val
@@ -168,6 +168,9 @@ class Options(object):
         self.extra_verbose = True
         util.extra_verbose = True
       elif key == '-p': self.profile_dest = val
+      elif key == '--help':
+        PrintUsage()
+        sys.exit(0)
 
     if not self.input:
       if 'GRIT_INPUT' in os.environ: