style: Simplify Owned FFI types.

And make them actually sound. We're defining functions on Rust-land that get
structs as arguments, but declaring them in C++ as getting pointers.

This is another step in order to be able to autogenerate ServoBindings.h and
remove bindings.rs altogether.

We remove FooOwned in favor of Owned<Foo>, which is generated via cbindgen.

It'd be good to actually mark Owned and such as MOZ_MUST_USE_TYPE, so I sent
https://github.com/eqrion/cbindgen/pull/307 for that.

Differential Revision: https://phabricator.services.mozilla.com/D24681

components/style/gecko_bindings/sugar/ownership.rs

@@ -320,27 +320,6 @@ impl<GeckoType> OwnedOrNull<GeckoType> {
         self.ptr.is_null()
     }
 
-    /// Returns an owned pointer if this is non-null, and `None` otherwise.
-    pub fn into_box_opt<ServoType>(self) -> Option<Box<ServoType>>
-    where
-        ServoType: HasBoxFFI<FFIType = GeckoType>,
-    {
-        if self.is_null() {
-            None
-        } else {
-            Some(unsafe { transmute(self) })
-        }
-    }
-
-    /// Returns an `Owned<GeckoType>` if non-null, `None` otherwise.
-    pub fn into_owned_opt(self) -> Option<Owned<GeckoType>> {
-        if self.is_null() {
-            None
-        } else {
-            Some(unsafe { transmute(self) })
-        }
-    }
-
     /// Gets a immutable reference to the underlying Gecko type, or `None` if
     /// null.
     pub fn borrow(&self) -> Option<&GeckoType> {