Update surfman to latest commit (#256)

Signed-off-by: Wu Wayne <yuweiwu@pm.me>

webxr/Cargo.toml

@@ -34,7 +34,7 @@ log = "0.4.6"
 openxr = { version = "0.19", optional = true }
 serde = { version = "1.0", optional = true }
 glow = "0.16"
-surfman = { git = "https://github.com/servo/surfman", rev = "c8d6b4b65aeab739ee7651602e29c8d58ceee123", features = [
+surfman = { git = "https://github.com/servo/surfman", rev = "300789ddbda45c89e9165c31118bf1c4c07f89f6", features = [
     "chains",
 ] }