Auto merge of #16550 - froydnj:gecko-bindings-gecko-debug, r=froydnj

switch gecko_bindings over to the gecko_debug feature [DO NOT MERGE]

...so that they use the correct Gecko structs regardless of whether Rust
code is being compiled with debug assertions or not.

This is the second part of the Servo-side changes for https://bugzilla.mozilla.org/show_bug.cgi?id=1357556

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

- [X] These changes do not require tests because building is a sufficient test.

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ports/geckolib/glue.rs

@@ -1992,7 +1992,7 @@ pub extern "C" fn Servo_GetComputedKeyframeValues(keyframes: RawGeckoKeyframeLis
 
 #[no_mangle]
 pub extern "C" fn Servo_AssertTreeIsClean(root: RawGeckoElementBorrowed) {
-    if !cfg!(debug_assertions) {
+    if !cfg!(feature = "gecko_debug") {
         panic!("Calling Servo_AssertTreeIsClean in release build");
     }