Mojo C++ bindings: make String16 and gfx::Size available in Blink

mojo/common/common_custom_types.mojom

Index: mojo/common/common_custom_types.mojom
diff --git a/mojo/common/common_custom_types.mojom b/mojo/common/common_custom_types.mojom
index 6c0f4eba773e7103215a1394ea31fc71fde0635b..7ece29bf46e5c63fa44cd41d2cbc54cfa7f26cd0 100644
--- a/mojo/common/common_custom_types.mojom
+++ b/mojo/common/common_custom_types.mojom
@@ -23,8 +23,13 @@ struct TimeDelta {
 [Native]
 struct TimeTicks;
 
-[Native]
-struct String16;
+// Corresponds to |base::string16| in base/strings/string16.h
+// Corresponds to |blink::WebString| in
+// third_party/WebKit/public/platform/WebString.h, and only accept non-null
+// 16-Bit |blink::WebString|.
+struct String16 {
+  array<uint16> data;
+};
 
 // Corresponds to |base::UnguessableToken| in base/unguessable_token.h
 struct UnguessableToken {