To reproduce:
1. Open project attached by QA on computer with OS X
2. Open "main" scene
3. Open profiler tab, enable record and deep profiling. Focus profiler on CPU
4. Enter play mode and hit Run Test button in game window
5. Inspect the profiler

Expected: WebRequestUtils.MakeInitialUrl should cost less time (now ~70ms), it shouldn't cause spikes.

Reproduced with: 5.5.0f3, 2017.1.1f1, 2017.2.0b10, 2017.3.0a7
OS: macOS Sierra 10.12.6 (16G29)

Note: Doesn't reproduce on windows (WebRequestUtils.MakeInitialUrl takes ~10ms)

Fixed in 2018.1.0a5
The issue was addressed by adding a new set of APIs to UnityWebRequest, that take instance on System.Uri instead of string URL.