Add a --nodownload option to get_toolchain_if_necessary.py

The Windows toolchain should be downloaded during gclient runhooks,
however because get_toolchain_if_necessary.py can either configure an
existing toolchain or download a toolchain there have been problems when
the toolchain is downloaded at an unexpected point. In particular, a
toolchain download during gn gen can lead to parallel downloads and
configuration failures.

The --nodownload option will be passed (after a subsequent change) by
build\vs_toolchain.py. A --nodownload option is needed instead of a more
positive --allowdownload option because of backwards compatibility. This
change will have no effect until the build\vs_toolchain.py change is
landed.

Bug: 584393, 662325
Change-Id: I9a0950e066744c0310e49e45d001a9113f1831cf
Reviewed-on: https://chromium-review.googlesource.com/c/1439805
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>

win_toolchain/get_toolchain_if_necessary.py

@@ -442,6 +442,8 @@ def main():
                     help='write information about toolchain to FILE')
   parser.add_option('--force', action='store_true',
                     help='force script to run on non-Windows hosts')
+  parser.add_option('--no-download', action='store_true',
+                    help='configure if present but don\'t download')
   parser.add_option('--toolchain-dir',
                     default=os.getenv(ENV_TOOLCHAIN_ROOT, BASEDIR),
                     help='directory to install toolchain into')
@@ -492,6 +494,10 @@ def main():
   # based on timestamps to make that case fast.
   current_hashes = CalculateToolchainHashes(target_dir, True)
   if desired_hash not in current_hashes:
+    if options.no_download:
+      raise SystemExit('Toolchain is out of date. Run "gclient runhooks" to '
+                      'update the toolchain, or set DEPOT_TOOLS_WIN_TOOLCHAIN=0'
+                      'to use the locally installed toolchain.')
     should_use_file = False
     should_use_http = False
     should_use_gs = False