Support CMake generated build files

Chromium does not use CMake, however ninja with Goma is used by LLVM
team.

Change-Id: I38349d0574048b22b32365e45c587e604e045442
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2572805
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>

autoninja.py

@@ -87,8 +87,14 @@ if os.path.exists(os.path.join(output_dir, 'args.gn')):
                    line_without_comment):
         use_remote_build = True
         continue
-elif os.path.exists(os.path.join(output_dir, 'rules.ninja')):
-  with open(os.path.join(output_dir, 'rules.ninja')) as file_handle:
+else:
+  for relative_path in [
+      '',  # GN keeps them in the root of output_dir
+      'CMakeFiles'
+  ]:
+    path = os.path.join(output_dir, relative_path, 'rules.ninja')
+    if os.path.exists(path):
+      with open(path) as file_handle:
         for line in file_handle:
           if re.match(r'^\s*command\s*=\s*\S+gomacc', line):
             use_remote_build = True