Add helpers for logging mojom objects.

mojo/common/BUILD.gn

Index: mojo/common/BUILD.gn
diff --git a/mojo/common/BUILD.gn b/mojo/common/BUILD.gn
index 25ef0caecd43bb73c8165d02f35e767889486c37..8bd451ad1bbbbc0442aeca369cb7d38c07a789ab 100644
--- a/mojo/common/BUILD.gn
+++ b/mojo/common/BUILD.gn
@@ -9,6 +9,7 @@ source_set("common") {
   sources = [
     "binding_set.h",
     "interface_ptr_set.h",
+    "logging.h",

[vardhan, 2015/12/11 00:12:20]

After looking at https://github.com/domokit/mojo/issues/480 and talking to
Trung..  I think the point of jamesr@'s comment on the github issue is to be
able to construct debug messages in a language agnostic way (by connecting and
querying a service).  Otherwise, we'll have to start maintaining printing
functions for each language.

I think it's better to hold off on making these mojom printing functions part of
mojo proper, and have them else where (inside your mojo/services/.. consumers?),
just so people don't start using it everywhere.

Maybe jamesr@ or vtl@ can comment if they have better suggestions

     "strong_binding_set.h",
   ]