Cleanup GN BUILD files, gni's and imports.

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
index f2631a0b466cfaf09e1e89c1693b704fac71391e..7b43cb33a39b1e130a122e0340ff097bf5fba249 100644
--- a/mojo/public/mojo.gni
+++ b/mojo/public/mojo.gni
@@ -27,8 +27,15 @@ if (defined(mojo_disable_dart_apptest_framework) &&
   mojo_use_dart_apptest_framework = false
 }
 
+# Embedder uses Mojo application code.
+mojo_use_application_in_sdk = true
+if (defined(mojo_disable_application_in_sdk) &&
+    mojo_disable_application_in_sdk) {
+  mojo_use_application_in_sdk = false
+}
+
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location
 # of the public SDK within a client repo.
-mojo_root = get_path_info("../..", "abspath")
+mojo_root = get_path_info("../../third_party/mojo/src/", "abspath")