GN: Remove dependency on //base for gmock.

tools/gn/secondary/testing/BUILD.gn

Index: tools/gn/secondary/testing/BUILD.gn
diff --git a/mojo/public/interfaces/shell/BUILD.gn b/tools/gn/secondary/testing/BUILD.gn
similarity index 67%
copy from mojo/public/interfaces/shell/BUILD.gn
copy to tools/gn/secondary/testing/BUILD.gn
index c11ae70a311389cec1c7e00da4de5ddc7e4de2df..2cafa68a281302c4e07352ce8fd1d63e4ee341dc 100644
--- a/mojo/public/interfaces/shell/BUILD.gn
+++ b/tools/gn/secondary/testing/BUILD.gn
@@ -2,10 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//mojo/public/tools/bindings/mojom.gni")
-
-mojom("shell") {
+source_set("gmock_mutant") {
   sources = [
-    "shell.mojom",
+    "gmock_mutant.h",  # gMock helpers
   ]
+
+  deps = [ "//base" ]
 }