Remove conditional for external_frontend

The old code was removed in crrev.com/c/chromium/src/+/1893272

DISABLE_THIRD_PARTY_CHECK=Build updates

Change-Id: If5bb1f6354a435624c6cd7e98914593518ca7fa5
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1942288
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index f78de66..7eb165f 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -41,6 +41,9 @@
 ]
 
 def _CheckChangesAreExclusiveToDirectory(input_api, output_api):
+    if input_api.change.DISABLE_THIRD_PARTY_CHECK != None:
+        return []
+
     def IsParentDir(file, dir):
         while file != '':
             if file == dir:
@@ -65,8 +68,10 @@
             num_in_dir = num_in_dir + 1
         if num_in_dir < num_affected:
             return [
-                output_api.PresubmitError(
-                    'CLs that affect files in "%s" should be limited to these files/directories.' % ', '.join(dirs))
+                output_api
+                .PresubmitError('CLs that affect files in "%s" should be limited to these files/directories.' +
+                                ' You can disable this check by adding DISABLE_THIRD_PARTY_CHECK=<reason> to your commit message' %
+                                ', '.join(dirs))
             ]
     return []