gclient: enable syntax validation by default (reland #1)

Original CL: https://chromium-review.googlesource.com/509451

Please pass --disable-syntax-validation as an escape hatch.

Bug: 570091
Change-Id: Ia9812506347685c31907225837ed50a0510952ee
Reviewed-on: https://chromium-review.googlesource.com/517950
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

gclient.py

@@ -2002,7 +2002,7 @@ def CMDsync(parser, args):
   parser.add_option('-m', '--manually_grab_svn_rev', action='store_true',
                     help='DEPRECATED: This is a no-op.')
   # TODO(phajdan.jr): Remove validation options once default (crbug/570091).
-  parser.add_option('--validate-syntax', action='store_true',
+  parser.add_option('--validate-syntax', action='store_true', default=True,
                     help='Validate the .gclient and DEPS syntax')
   parser.add_option('--disable-syntax-validation', action='store_false',
                     dest='validate_syntax',