Make mojo_system static and mojo_system_impl a component, never use both

mojo/system/core_test_base.h

Index: mojo/system/core_test_base.h
diff --git a/mojo/system/core_test_base.h b/mojo/system/core_test_base.h
index beb985b228923df94a7e9e2c3d8b18507f2ceeb3..b7259380950f5ab8eb3b6ac5532e1319d198f5c9 100644
--- a/mojo/system/core_test_base.h
+++ b/mojo/system/core_test_base.h
@@ -14,7 +14,7 @@
 namespace mojo {
 namespace system {
 
-class CoreImpl;
+class Core;
 
 namespace test {
 
@@ -34,10 +34,10 @@ class CoreTestBase : public testing::Test {
   // |info| must remain alive until the returned handle is closed.
   MojoHandle CreateMockHandle(MockHandleInfo* info);
 
-  CoreImpl* core() { return core_; }
+  Core* core() { return core_; }
 
  private:
-  CoreImpl* core_;
+  Core* core_;
 
   DISALLOW_COPY_AND_ASSIGN(CoreTestBase);
 };