Remove redundant rebase_path from gn file

rebase_path is being applied twice on skia_workaround_list file path.
This patch removes the redundancy.

Component: Gpu config Gn
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: Ieea6c860dabbd0d76983a67a7fa7de53c014a963
Reviewed-on: https://chromium-review.googlesource.com/1028578Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553831}

gpu/config/BUILD.gn

@@ -72,7 +72,7 @@ action("workaround_list") {
   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),
+    skia_workaround_list,
   ]
 
   outputs = [