Add "none" option for --pr, to force software path renderer

There was no way to get this behavior without triggering an error before.

Bug: skia:
Change-Id: Id9cb9090e47226dadad0032f06ec205dd2538dcc
Reviewed-on: https://skia-review.googlesource.com/37001
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/tools/flags/SkCommonFlagsPathRenderer.h b/tools/flags/SkCommonFlagsPathRenderer.h
index ac293a1..0a53beb 100644
--- a/tools/flags/SkCommonFlagsPathRenderer.h
+++ b/tools/flags/SkCommonFlagsPathRenderer.h
@@ -46,6 +46,8 @@
         return GpuPathRenderers::kTessellating;
     } else if (!strcmp(name, "grdefault")) {
         return GpuPathRenderers::kDefault;
+    } else if (!strcmp(name, "none")) {
+        return GpuPathRenderers::kNone;
     }
     SK_ABORT(SkStringPrintf("error: unknown named path renderer \"%s\"\n", name).c_str());
     return GpuPathRenderers::kNone;