Mojo C++ bindings: move support for mojo::String.

mojo/public/cpp/bindings/string.h

Index: mojo/public/cpp/bindings/string.h
diff --git a/mojo/public/cpp/bindings/string.h b/mojo/public/cpp/bindings/string.h
index e02f089a7cd98144d1396b7421b593b898818de9..5f22b4cec60e50a3dea525fbd2770a51a93cbd6b 100644
--- a/mojo/public/cpp/bindings/string.h
+++ b/mojo/public/cpp/bindings/string.h
@@ -38,6 +38,9 @@ class String {
   String(const char chars[N])
       : value_(chars, N - 1), is_null_(false) {}
 
+  String(std::string&& other) : value_(std::move(other)), is_null_(false) {}
+  String(String&& other) : is_null_(true) { Swap(&other); }
+
   template <typename U>
   static String From(const U& other) {
     return TypeConverter<String, U>::Convert(other);
@@ -68,6 +71,18 @@ class String {
     return *this;
   }
 
+  String& operator=(std::string&& other) {
+    value_ = std::move(other);
+    is_null_ = false;
+    return *this;
+  }
+  String& operator=(String&& other) {
+    is_null_ = true;
+    value_.clear();
+    Swap(&other);
+    return *this;
+  }
+
   bool is_null() const { return is_null_; }
 
   size_t size() const { return value_.size(); }
@@ -80,6 +95,16 @@ class String {
   const std::string& get() const { return value_; }
   operator const std::string&() const { return value_; }
 
+  // Returns a const reference to the |std::string| managed by this class. If
+  // the string is null, this will be an empty std::string.
+  const std::string& storage() const { return value_; }
+
+  // Passes the underlying storage and resets this string to null.
+  std::string PassStorage() {
+    is_null_ = true;
+    return std::move(value_);
+  }
+
   void Swap(String* other) {
     std::swap(is_null_, other->is_null_);
     value_.swap(other->value_);