Fix the build for NLL

Test with `RUSTFLAGS="-Zborrowck=mir -Ztwo-phase-borrows" cargo build`

https://internals.rust-lang.org/t/help-us-get-non-lexical-lifetimes-nll-over-the-finish-line/7807/7

components/script/dom/imagedata.rs

@@ -65,7 +65,7 @@ impl ImageData {
         if let Some(jsobject) = opt_jsobject {
             let cx = global.get_cx();
             typedarray!(in(cx) let array_res: Uint8ClampedArray = jsobject);
-            let mut array = array_res
+            let array = array_res
                 .map_err(|_| Error::Type("Argument to Image data is not an Uint8ClampedArray".to_owned()))?;
 
             let byte_len = array.as_slice().len() as u32;