[blink] Ignore webidl2 of WPT in PRESUBMIT checks

The new exception is added in preparation of:
https://github.com/web-platform-tests/wpt/pull/16864

Also remove the unnecessary slashes from the regexes: '\' does not need
to be escaped again in Python raw strings.

Change-Id: I3453468b138f40e2adf1c7aaffa0076fbdb5f649
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636390
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665527}

third_party/blink/PRESUBMIT.py

@@ -25,9 +25,10 @@ except IOError:
 
 
 _EXCLUDED_PATHS = (
-    # These directories are created and updated via a script.
-    r'^third_party[\\\/]blink[\\\/]tools[\\\/]blinkpy[\\\/]third_party[\\\/]wpt[\\\/]wpt[\\\/].*',
-    r'^third_party[\\\/]blink[\\\/]web_tests[\\\/]external[\\\/]wpt[\\\/]tools[\\\/].*',
+    # These are third-party dependencies that we don't directly control.
+    r'^third_party[\\/]blink[\\/]tools[\\/]blinkpy[\\/]third_party[\\/]wpt[\\/]wpt[\\/].*',
+    r'^third_party[\\/]blink[\\/]web_tests[\\/]external[\\/]wpt[\\/]tools[\\/].*',
+    r'^third_party[\\/]blink[\\/]web_tests[\\/]external[\\/]wpt[\\/]resources[\\/]webidl2[\\/].*',
 )