Add explicit {copy,move} {ctor,assignment operator} to WTF::String

Change-Id: I28260c2656d1494c9bfb690f8486eacbeb44a53a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2296638
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810795}

third_party/blink/renderer/platform/wtf/text/wtf_string.h

@@ -103,6 +103,14 @@ class WTF_EXPORT String {
   String(StringImpl* impl) : impl_(impl) {}
   String(scoped_refptr<StringImpl> impl) : impl_(std::move(impl)) {}
 
+  // Copying a String is a relatively inexpensive, since the underlying data is
+  // immutable and refcounted.
+  String(const String&) = default;
+  String& operator=(const String&) = default;
+
+  String(String&&) noexcept = default;
+  String& operator=(String&&) = default;
+
   void swap(String& o) { impl_.swap(o.impl_); }
 
   template <typename CharType>