Auto merge of #19440 - asajeffrey:test-perf-utf8, r=jdm

Added explicit UTF-8 encoding to test-perf.

<!-- Please describe your changes on the following line: -->

Added explicit UTF-8 encoding to test-perf, to try to avoid:
```
Traceback (most recent call last):
  File "runner.py", line 371, in <module>
    main()
  File "runner.py", line 361, in main
    save_result_csv(results, args.output_file, testcases, args.runs, args.base)
  File "runner.py", line 287, in save_result_csv
    writer.writerows(results)
  File "/usr/lib/python3.4/csv.py", line 159, in writerows
    return self.writer.writerows(rows)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 101-107: ordinal not in range(128)
```
http://build.servo.org/builders/linux-nightly/builds/526/steps/test/logs/stdio

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is test infrastructure

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etc/ci/performance/runner.py

@@ -100,7 +100,7 @@ def parse_log(log, testcase, url):
     block = []
     copy = False
     for line_bytes in log.splitlines():
-        line = line_bytes.decode()
+        line = line_bytes.decode('utf-8')
 
         if line.strip() == ("[PERF] perf block start"):
             copy = True
@@ -281,7 +281,7 @@ def save_result_csv(results, filename, manifest, expected_runs, base):
         'unloadEventStart',
     ]
 
-    with open(filename, 'w') as csvfile:
+    with open(filename, 'w', encoding='utf-8') as csvfile:
         writer = csv.DictWriter(csvfile, fieldnames)
         writer.writeheader()
         writer.writerows(results)