Add field-initializing constructors to generated mojo structs.

mojo/public/cpp/bindings/tests/equals_unittest.cc

Index: mojo/public/cpp/bindings/tests/equals_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/equals_unittest.cc b/mojo/public/cpp/bindings/tests/equals_unittest.cc
index 4862b93f3a22888327a97de3fed28fe8ed8d5144..036d49c317069e598e4b6058b23844b3508b66cd 100644
--- a/mojo/public/cpp/bindings/tests/equals_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/equals_unittest.cc
@@ -14,12 +14,7 @@ namespace test {
 namespace {
 
 RectPtr CreateRect() {
-  RectPtr r = Rect::New();
-  r->x = 1;
-  r->y = 2;
-  r->width = 3;
-  r->height = 4;
-  return r;
+  return Rect::New(1, 2, 3, 4);
 }
 
 using EqualsTest = testing::Test;
@@ -48,9 +43,7 @@ TEST_F(EqualsTest, Struct) {
 }
 
 TEST_F(EqualsTest, StructNested) {
-  RectPairPtr p1(RectPair::New());
-  p1->first = CreateRect();
-  p1->second = CreateRect();
+  RectPairPtr p1(RectPair::New(CreateRect(), CreateRect()));
   RectPairPtr p2(p1.Clone());
   EXPECT_TRUE(p1.Equals(p2));
   p2->second->width = 0;
@@ -60,8 +53,7 @@ TEST_F(EqualsTest, StructNested) {
 }
 
 TEST_F(EqualsTest, Array) {
-  NamedRegionPtr n1(NamedRegion::New());
-  n1->name.emplace("n1");
+  NamedRegionPtr n1(NamedRegion::New(std::string("n1")));
   n1->rects.emplace();
   n1->rects->push_back(CreateRect());
   NamedRegionPtr n2(n1.Clone());