clean up GN targets that are already migrated, add pepper_hash_for_uma.

ppapi/BUILD.gn

Index: ppapi/BUILD.gn
diff --git a/ppapi/BUILD.gn b/ppapi/BUILD.gn
index 9bc9e889a142c3ef8cc095f64c0badaff66a7851..7cf8e967b79bc708d92da32d0ab1f4693867d35c 100644
--- a/ppapi/BUILD.gn
+++ b/ppapi/BUILD.gn
@@ -6,10 +6,10 @@ copy("copy_test_files") {
   visibility = [ ":*" ]
   sources = [
     # Keep "test_case.html.mock-http-headers" with "test_case.html".
+    "tests/ppapi_nacl_tests_newlib.nmf",
     "tests/test_case.html",
     "tests/test_case.html.mock-http-headers",
     "tests/test_page.css",
-    "tests/ppapi_nacl_tests_newlib.nmf",
   ]
   outputs = [
     "$root_out_dir/{{source_file_part}}",
@@ -116,3 +116,13 @@ test("ppapi_perftests") {
     "//testing/gtest",
   ]
 }
+
+executable("pepper_hash_for_uma") {
+  sources = [
+    "tools/pepper_hash_for_uma.cc",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}