[Benchmarking] Gtest process provide non-unicode executable name.

Speculative fix. See bug for details.

Bug: 1039019
Change-Id: I15ee33cfba36b1896b089147fd8fa6f6cd9e2459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986969
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728353}

testing/scripts/run_performance_tests.py

@@ -189,7 +189,7 @@ class GtestCommandGenerator(object):
     return self._override_executable or self._options.executable
 
   def _get_executable(self):
-    executable = self.executable_name
+    executable = str(self.executable_name)
     if IsWindows():
       return r'.\%s.exe' % executable
     else: