Create new target to hold gpu test code, enable direct testing of GrEffects in GM.

gm/gmmain.cpp

Index: gm/gmmain.cpp
diff --git a/gm/gmmain.cpp b/gm/gmmain.cpp
index 4db3a0d24272d26c0c4a59f31a0933cac0e86045..eebaefe2cfbf362251867ad2737417ad5ad6a224 100644
--- a/gm/gmmain.cpp
+++ b/gm/gmmain.cpp
@@ -1558,8 +1558,8 @@ ErrorCombination run_multiple_configs(GMMain &gmmain, GM *gm,
             errorsForAllConfigs.add(kIntentionallySkipped_ErrorType);
             continue;
         }
-        if ((gmFlags & GM::kSkipGPU_Flag) &&
-            kGPU_Backend == config.fBackend) {
+        if (((gmFlags & GM::kSkipGPU_Flag) && kGPU_Backend == config.fBackend) ||
+            ((gmFlags & GM::kGPUOnly_Flag) && kGPU_Backend != config.fBackend)) {
             gmmain.RecordTestResults(kIntentionallySkipped_ErrorType, shortNamePlusConfig,
                                      renderModeDescriptor);
             errorsForAllConfigs.add(kIntentionallySkipped_ErrorType);