mojo/public: Include gtest.h as "gtest/gtest.h", instead of via "absolute" path.

mojo/public/mojo_sdk.gni

Index: mojo/public/mojo_sdk.gni
diff --git a/mojo/public/mojo_sdk.gni b/mojo/public/mojo_sdk.gni
index 716c2c3a744149dd8a7057bc85dfa373eee5976a..c84fc0fdb14019d5a322800e6283d2665c2e0b83 100644
--- a/mojo/public/mojo_sdk.gni
+++ b/mojo/public/mojo_sdk.gni
@@ -115,9 +115,8 @@ template("mojo_sdk_source_set") {
           # the Mojo SDK should be on targets within the same file or on a
           # whitelisted set of external dependencies.
           dep_dir = get_path_info(dep, "dir")
-          assert(dep_dir == "." || dep == "//testing/gtest" ||
-                 (dep == "//dart/runtime:libdart" &&
-                  target_name == "mojo_internal_impl"))
+          assert(dep_dir == "." || (dep == "//dart/runtime:libdart" &&
+                                    target_name == "mojo_internal_impl"))
         }
         deps += [ dep ]
       }