Fix test to work in checkouts not named 'chromium'.

Updates expand_owners_unittest.py to work even if the
checkout does not begin with a 'chromium' dir.

Note: This does not fix the issue that makes this test
fail on bots yet and so doesn't re-add it to the list of
tests ran by metrics_python_tests.py. This is a slightly
different issue, where on bots, there isn't even a 'src'
directory, which trips up other assumptions in the test.

Bug: 1126653
Change-Id: I0f345f3efa3a54b4f7ba08106fd6d08ffbc5345b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505573
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#821875}

tools/metrics/histograms/expand_owners.py

@@ -142,10 +142,10 @@ def _GetOwnersFilePath(path):
 
     return os.path.abspath(
         os.path.join(*(_DIR_ABOVE_TOOLS + path_without_src.split(os.sep))))
-  else:
-    raise Error('The given path {} is not well-formatted.'
-                'Well-formatted paths begin with "src/" and end with "OWNERS"'
-                .format(path))
+
+  raise Error(
+      'The given path {} is not well-formatted. Well-formatted paths begin '
+      'with "src/" and end with "OWNERS"'.format(path))
 
 
 def _ExtractEmailAddressesFromOWNERS(path, depth=0):

tools/metrics/histograms/expand_owners_unittest.py

@@ -13,6 +13,19 @@ import xml.dom.minidom
 _DEFAULT_COMPONENT = '# COMPONENT: Default>Component'
 
 
+def _DirnameN(path, n):
+  """Calls os.path.dirname() on the argument n times."""
+  path = os.path.abspath(path)
+  for _ in range(n):
+    path = os.path.dirname(path)
+  return path
+
+
+assert __file__.endswith('tools/metrics/histograms/expand_owners_unittest.py')
+
+_PATH_TO_CHROMIUM_DIR = _DirnameN(__file__, 5)
+
+
 def _GetFileDirective(path):
   """Returns a file directive line.
 
@@ -31,9 +44,12 @@ def _GetSrcRelativePath(path):
 
   Args:
     path: An absolute path, e.g. '/some/directory/chromium/src/tools/OWNERS'.
+
+  Returns:
+    A src-relative path, e.g.'src/tools/OWNERS'.
   """
-  # TODO(crbug/1126653): This fails if chromium is not in the path.
-  return path.split('chromium/')[1]
+  assert path.startswith(_PATH_TO_CHROMIUM_DIR)
+  return path[len(_PATH_TO_CHROMIUM_DIR) + 1:]
 
 
 def _MakeOwnersFile(filename, directory):