gpu: Cleanup 'gn check' errors for gpu_unittests target.

Tested on Linux with the following command lines:

$ gn gen out-gn/Debug
$ gn check out-gn/Debug //gpu:*

BUG=367595, 376000
TEST=see above
R=sievers@chromium.org,dpranke@chromium.org
TBR=brettw@chromium.org # for .gn

Review URL: https://codereview.chromium.org/1136283008

Cr-Commit-Position: refs/heads/master@{#330165}

.gn

@@ -45,6 +45,8 @@ check_targets = [
   "//google_update/*",
 
   #"//gpu/*",  # Lots of errors.
+  "//gpu:*",
+
   #"//ios/*",
   "//ipc/*",
 

gpu/BUILD.gn

@@ -241,9 +241,10 @@ test("gpu_unittests") {
     "//testing/gtest",
     "//third_party/angle:translator",
     "//ui/gfx",
-    "//ui/gfx:test_support",
     "//ui/gfx/geometry",
+    "//ui/gfx:test_support",
     "//ui/gl",
+    "//ui/gl:gl_unittest_utils",
     "//gpu/command_buffer/common:gles2_utils",
     "//gpu/command_buffer/client:gles2_c_lib",
     "//gpu/command_buffer/client:gles2_implementation",