"typedef Foo Bar" -> "using Bar = Foo" in //mojo/edk/....

The latter is now standard, and to be consistent (with new things I'll
add), I guess I should convert everything.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1154903003
diff --git a/mojo/edk/system/core_unittest.cc b/mojo/edk/system/core_unittest.cc
index b789301..c562097 100644
--- a/mojo/edk/system/core_unittest.cc
+++ b/mojo/edk/system/core_unittest.cc
@@ -24,7 +24,7 @@
                                       MOJO_HANDLE_SIGNAL_WRITABLE |
                                       MOJO_HANDLE_SIGNAL_PEER_CLOSED;
 
-typedef test::CoreTestBase CoreTest;
+using CoreTest = test::CoreTestBase;
 
 TEST_F(CoreTest, GetTimeTicksNow) {
   const MojoTimeTicks start = core()->GetTimeTicksNow();