Add flag to force gpu trace markers to be used and printed out for debugging.

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 8e892ee5a2d2ca7b27655b6dfa2286fa22fc06da..b7901400fbd68c132c4b448e20874b9bb64ba291 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -2778,13 +2778,21 @@ void GrGLGpu::didAddGpuTraceMarker() {
     if (this->caps()->gpuTracingSupport()) {
         const GrTraceMarkerSet& markerArray = this->getActiveTraceMarkers();
         SkString markerString = markerArray.toStringLast();
+#if GR_FORCE_GPU_TRACE_DEBUGGING
+        SkDebugf("%s\n", markerString.c_str());
+#else
         GL_CALL(PushGroupMarker(0, markerString.c_str()));
+#endif
     }
 }
 
 void GrGLGpu::didRemoveGpuTraceMarker() {
     if (this->caps()->gpuTracingSupport()) {
+#if GR_FORCE_GPU_TRACE_DEBUGGING
+        SkDebugf("Pop trace marker.\n");
+#else
         GL_CALL(PopGroupMarker());
+#endif
     }
 }