gpu: merge skia workaround names with gpu workarounds

Now that Skia has its own list of workaround names, build the list of
workarounds from a combined list of Skia's and Chrome's.

Bug: 829614
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: I2d33190fba17421799aed6786f1e675a37601aed
Reviewed-on: https://chromium-review.googlesource.com/1026804Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553387}

gpu/config/BUILD.gn

@@ -69,8 +69,10 @@ action("process_json") {
 action("workaround_list") {
   script = "build_workaround_header.py"
 
+  skia_workaround_list = "//third_party/skia/src/gpu/gpu_workaround_list.txt"
   inputs = [
     "gpu_workaround_list.txt",
+    rebase_path(skia_workaround_list, root_build_dir),
   ]
 
   outputs = [