[Ozone-GN] Fix GN build

The name of the file for OzoneGpuTestHelper was incorrect.

BUG=none
TEST=Built Ozone with a GN build

Review URL: https://codereview.chromium.org/978473002

Cr-Commit-Position: refs/heads/master@{#318944}

ui/ozone/BUILD.gn

@@ -94,8 +94,8 @@ component("ozone") {
     "platform_selection.h",
     "public/input_controller.cc",
     "public/input_controller.h",
-    "public/ozone_gpu_thread_helper.cc",
-    "public/ozone_gpu_thread_helper.h",
+    "public/ozone_gpu_test_helper.cc",
+    "public/ozone_gpu_test_helper.h",
     "public/ozone_platform.cc",
     "public/ozone_platform.h",
     "public/ozone_switches.cc",