Rename XRFrameOfReferenceType enums

The values of the enum was updated in the spec[1].

[1] https://github.com/immersive-web/webxr/pull/370

Bug: 852520
Change-Id: I916943bee355936e7430aa4e36d0eeb6b76607d5
Reviewed-on: https://chromium-review.googlesource.com/1101118
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Byoungkwon Ko <codeimpl@gmail.com>
Cr-Commit-Position: refs/heads/master@{#567340}
diff --git a/interfaces/webxr.idl b/interfaces/webxr.idl
index b409009..45c0149 100644
--- a/interfaces/webxr.idl
+++ b/interfaces/webxr.idl
@@ -60,8 +60,8 @@
 };
 
 enum XRFrameOfReferenceType {
-  "headModel",
-  "eyeLevel",
+  "head-model",
+  "eye-level",
   "stage",
 };