Add dependency gpu:test_support -> ui/gfx:test_support

In some jumbo build configurations the linker needs
gfx::PrintTo(gfx::Size const&, ...) referenced from
obj/gpu/libtest_support.a
testing::internal::UniversalPrinter<gfx::Size>::Print(
    gfx::Size const&, ...)

This reference might be in dead code that would otherwise (in
non-jumbo builds) have been stripped earlier, but since the
reference apparently is there it's best to have
the dependency as well.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I6fe98a5427e5bc30625773bfa73d5b248b34f30a
Reviewed-on: https://chromium-review.googlesource.com/1005340Reviewed-by: Victor Miura <vmiura@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#562997}

gpu/BUILD.gn

@@ -171,6 +171,7 @@ static_library("test_support") {
     "//gpu/skia_bindings:skia_bindings",
     "//testing/gmock",
     "//testing/gtest",
+    "//ui/gfx:test_support",
     "//ui/gl:gl_unittest_utils",
     "//ui/gl:test_support",
   ]