LibWeb: Use "= default" to declare empty constructors and descructors

A types which have special functions declared with "= default can be
trivially copied. Besides being good practice, the compiler might be
able generate copy and initialize code in a more optimized way.

Found By PVS-Studio: https://pvs-studio.com/en/docs/warnings/v832/

Userland/Libraries/LibWeb/CSS/MediaQuery.h

@@ -20,7 +20,7 @@ class MediaQuery : public RefCounted<MediaQuery> {
     friend class Parser;
 
 public:
-    ~MediaQuery() { }
+    ~MediaQuery() = default;
 
     // https://www.w3.org/TR/mediaqueries-4/#media-types
     enum class MediaType {
@@ -82,7 +82,7 @@ public:
     String to_string() const;
 
 private:
-    MediaQuery() { }
+    MediaQuery() = default;
 
     // https://www.w3.org/TR/mediaqueries-4/#mq-not
     bool m_negated { false };

Userland/Libraries/LibWeb/DOM/Window.cpp

@@ -31,7 +31,7 @@ public:
         , m_handler(move(handler))
     {
     }
-    ~RequestAnimationFrameCallback() { }
+    ~RequestAnimationFrameCallback() = default;
 
     i32 id() const { return m_id; }
     bool is_cancelled() const { return !m_handler; }