Fix PRESUBMIT.py for web_features.mojom enums.xml check on Windows

The output XML contains a relative path to the mojom file. On Windows,
the expected path contains '\' separators, but the actual contains '/'.
tools/metrics/histograms/update_use_counter_feature_enum.py will update
the enums.xml file, but uses a hardcoded path.  The fix is to use a
hardcoded path for
third_party/blink/public/mojom/web_feature/PRESUBMIT.py as well.

Bug: 966496
Change-Id: Iad605bea175ee86abe7bb8c160ae2be30bf8f65e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627636Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666311}

third_party/blink/public/mojom/web_feature/PRESUBMIT.py

@@ -21,14 +21,13 @@ def _RunUmaHistogramChecks(input_api, output_api):  # pylint: disable=C0103
     finally:
         sys.path = original_sys_path
 
-    source_path = ''
     for f in input_api.AffectedFiles():
         if f.LocalPath().endswith('web_feature.mojom'):
-            source_path = f.LocalPath()
             break
     else:
         return []
 
+    source_path = 'third_party/blink/public/mojom/web_feature/web_feature.mojom'
     start_marker = '^enum WebFeature {'
     end_marker = '^kNumberOfFeatures'
     presubmit_error = update_histogram_enum.CheckPresubmitErrors(