[Telemetry] Add support for macOS 10.13 High Sierra.

telemetry/telemetry/internal/platform/mac_platform_backend.py

Index: telemetry/telemetry/internal/platform/mac_platform_backend.py
diff --git a/telemetry/telemetry/internal/platform/mac_platform_backend.py b/telemetry/telemetry/internal/platform/mac_platform_backend.py
index ce4ac7699fbf7bd183ada44413bbc23a1b88cba3..2e434bd587ef2a1203b49df347afa87b7415f12d 100644
--- a/telemetry/telemetry/internal/platform/mac_platform_backend.py
+++ b/telemetry/telemetry/internal/platform/mac_platform_backend.py
@@ -160,6 +160,8 @@ class MacPlatformBackend(posix_platform_backend.PosixPlatformBackend):
       return os_version_module.ELCAPITAN
     if os_version.startswith('16.'):
       return os_version_module.SIERRA
+    if os_version.startswith('17.'):
+      return os_version_module.HIGHSIERRA
 
     raise NotImplementedError('Unknown mac version %s.' % os_version)