clippy: Fix default_constructed_unit_structs warnings in components (servo#31827)

* clippy: Fix default constructed unit structs warnings

* refactor: Allow default constructed unit structs to avoid gfx build error

Author: oluwatobiss

components/gfx/font_cache_thread.rs

@@ -491,6 +491,7 @@ impl FontCacheThread {
                 // TODO: Allow users to specify these.
                 let generic_fonts = populate_generic_fonts();
 
+                #[allow(clippy::default_constructed_unit_structs)]
                 let mut cache = FontCache {
                     port,
                     channel_to_self,

components/gfx/font_context.rs

@@ -65,6 +65,7 @@ pub struct FontContext<S: FontSource> {
 
 impl<S: FontSource> FontContext<S> {
     pub fn new(font_source: S) -> FontContext<S> {
+        #[allow(clippy::default_constructed_unit_structs)]
         let handle = FontContextHandle::default();
         FontContext {
             platform_handle: handle,

components/script/dom/bindings/buffer_source.rs

@@ -77,7 +77,7 @@ where
     let heap_buffer_source = match init {
         HeapTypedArrayInit::Buffer(buffer_source) => HeapBufferSource {
             buffer_source,
-            phantom: PhantomData::default(),
+            phantom: PhantomData,
         },
         HeapTypedArrayInit::Info { len, cx } => {
             rooted!(in (*cx) let mut array = ptr::null_mut::<JSObject>());
@@ -125,7 +125,7 @@ where
     pub fn default() -> HeapBufferSource<T> {
         HeapBufferSource {
             buffer_source: BufferSource::Default(Box::new(Heap::default())),
-            phantom: PhantomData::default(),
+            phantom: PhantomData,
         }
     }
 
@@ -436,7 +436,7 @@ where
 
         HeapBufferSource {
             buffer_source: BufferSource::ArrayBuffer(Heap::boxed(*array_buffer)),
-            phantom: PhantomData::default(),
+            phantom: PhantomData,
         }
     }
 }