gdb/blink.py Fix string encoding error

ustring_to_string attempts to concatenate an error message, which is a
string, with the result of a string encoded into utf-8, which is bytes.
For this to work in Python3 they need to be the same type so ensure we
also encode the error message.

Change-Id: I2591784641bb38eec0c168080acc84230f4932a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4790023
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1185149}

third_party/blink/tools/gdb/blink.py

@@ -70,7 +70,7 @@ def ustring_to_string(ptr, length=None):
     char_vals = [int((ptr + i).dereference()) for i in range(length)]
     string = struct.pack('H' * length, *char_vals).decode(
         'utf-16', 'replace').encode('utf-8')
-    return string + error_message
+    return string + error_message.encode('utf-8')
 
 
 def lstring_to_string(ptr, length=None):