Use gpu::Mailbox in IPCs instead of std::string

cc/texture_layer_unittest.cc

Index: cc/texture_layer_unittest.cc
diff --git a/cc/texture_layer_unittest.cc b/cc/texture_layer_unittest.cc
index f7c521c78c8170affeafa2cb037bea6b75644dbb..24df2033493cb1fd3ba9eee920951d992b49262a 100644
--- a/cc/texture_layer_unittest.cc
+++ b/cc/texture_layer_unittest.cc
@@ -277,8 +277,10 @@ class TextureLayerImplWithMailboxThreadedCallback : public ThreadedTest {
   }
 
   void SetMailbox(char mailbox_char) {
+    gpu::Mailbox name;
+    memset(name.name, mailbox_char, sizeof(name.name));
     TextureMailbox mailbox(
-        std::string(64, mailbox_char),
+        name,
         base::Bind(
             &TextureLayerImplWithMailboxThreadedCallback::ReleaseCallback,
             base::Unretained(this)));