headless: Increase error limit for render test screenshots

Some tests are flaky, let's see if this helps.

Bug: 830823
Change-Id: I48f0c8284ee8f94518bb4cc4c955fae3edddcdf4
TBR: alexclarke@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1005006Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549833}

headless/test/headless_render_test.cc

@@ -137,7 +137,7 @@ bool ScreenshotMatchesGolden(const std::string& screenshot_data,
   if (expected_bitmap.empty())
     return false;
 
-  return MatchesBitmap(expected_bitmap, actual_bitmap, 0);
+  return MatchesBitmap(expected_bitmap, actual_bitmap, 5);
 }
 
 }  // namespace