EDK: Move ref counting classes to mojo/edk/util.

EDK: Move ref counting classes to mojo/edk/util.

(In particular, RefPtr<T> and RefCountedThreadSafe<T>.)

The embedder API will use RefPtr<T> in its interface (replacing the
current use of scoped_refptr<T>), and we don't want to expose things in
mojo/edk/system to embedders.

Note: As a general rule, in .cc files in mojo/edk/system, I always used
|using mojo::util::RefPtr| (since |util::RefPtr| is very tiresome; I did
it everywhere for consistency).

R=vardhan@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/6a1f1a8187d874da20c5daf21e438c8c8492ec91

[vardhan, 2015-10-30 22:07:35]

lgtm

[viettrungluu, 2015-10-30 22:18:06]

Committed patchset #1 (id:1) manually as
6a1f1a8187d874da20c5daf21e438c8c8492ec91 (presubmit successful).