autotest.py: Use autoninja from PATH. Make -C optional

Use autoninja from PATH so that it matches the version used when
building explicitly (allows for devs that are using a custom-built
version of ninja).

Allow omitting -C when script is run from within the output directory.
This matches ninja's behavior.

Bug: None
Change-Id: I55763f178e2cf85892efbb4397a7ec81ed054aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419608
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808875}

tools/autotest.py

@@ -117,7 +117,8 @@ def RunCommand(cmd, **kwargs):
 
 def BuildTestTargetsWithNinja(out_dir, targets, dry_run):
   """Builds the specified targets with ninja"""
-  ninja_path = os.path.join(DEPOT_TOOLS_DIR, 'autoninja')
+  # Use autoninja from PATH to match version used for manual builds.
+  ninja_path = 'autoninja'
   if sys.platform.startswith('win32'):
     ninja_path += '.bat'
   cmd = [ninja_path, '-C', out_dir] + targets
@@ -349,12 +350,10 @@ def BuildTestFilter(filenames, line):
 def main():
   parser = argparse.ArgumentParser(
       description=__doc__, formatter_class=argparse.RawTextHelpFormatter)
-  parser.add_argument(
-      '--out-dir',
-      '-C',
-      metavar='OUT_DIR',
-      help='output directory of the build',
-      required=True)
+  parser.add_argument('--out-dir',
+                      '-C',
+                      metavar='OUT_DIR',
+                      help='output directory of the build')
   parser.add_argument(
       '--run-all',
       action='store_true',
@@ -375,6 +374,10 @@ def main():
 
   args, _extras = parser.parse_known_args()
 
+  # Use CWD as out_dir when build.ninja exists.
+  if not args.out_dir and os.path.exists('build.ninja'):
+    args.out_dir = '.'
+
   if not os.path.isdir(args.out_dir):
     parser.error(f'OUT_DIR "{args.out_dir}" does not exist.')
   target_cache = TargetCache(args.out_dir)