Change mojo application name to {name}.mojo

Also remove obsolete reference to gyp files.

R=abarth@chromium.org, brettw@chromium.org, aa@chromium.org

Review URL: https://codereview.chromium.org/713513002
diff --git a/sky/tools/debugger/BUILD.gn b/sky/tools/debugger/BUILD.gn
index 72a4a1b..91e581f 100644
--- a/sky/tools/debugger/BUILD.gn
+++ b/sky/tools/debugger/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//mojo/public/mojo_application.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 group("debugger") {
@@ -13,7 +14,7 @@
   ]
 }
 
-shared_library("sky_debugger") {
+mojo_native_application("sky_debugger") {
   sources = [
     "debugger.cc",
     "debugger.h",