check_blink_style: Fix two issues in presubmit

* PRESUBMIT.py:
  Decode stderr output to avoid to print b'...' to the console.

* cpp.py:
  Fix "FutureWarning: Possible nested set at position ..."

Bug: 1207012
Change-Id: Id9672dabcd902cd8678b80baf5a1e399c19b0425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3858989
Auto-Submit: Kent Tamura <[email protected]>
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Yuki Shiino <[email protected]>
Commit-Queue: Kent Tamura <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1040261}
diff --git a/third_party/blink/PRESUBMIT.py b/third_party/blink/PRESUBMIT.py
index 7de25ab6..5805bba 100644
--- a/third_party/blink/PRESUBMIT.py
+++ b/third_party/blink/PRESUBMIT.py
@@ -194,7 +194,7 @@
             if child.returncode != 0:
                 results.append(
                     output_api.PresubmitError('check_blink_style.py failed',
-                                              [stderrdata]))
+                                              [stderrdata.decode('utf-8')]))
         except Exception as e:
             results.append(
                 output_api.PresubmitNotifyResult(