Add a message when an op list fails to instantiate

From local testing, TSAN causes this to happen for 40-50 GMs in the
middle of a DM run. Having this message would have saved me a couple
hours of head-scratching and debugging.

Best theory about the TSAN thing is that it interferes with a driver
thread that's reclaiming freed resources, but that's just a guess.

Bug: skia:
Change-Id: Ie6759fd2f22953595920cb271e2317c5ed1a0941
Reviewed-on: https://skia-review.googlesource.com/37441
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrDrawingManager.cpp b/src/gpu/GrDrawingManager.cpp
index 22594e7..d7ab495 100644
--- a/src/gpu/GrDrawingManager.cpp
+++ b/src/gpu/GrDrawingManager.cpp
@@ -170,6 +170,7 @@
 
     for (int i = 0; i < fOpLists.count(); ++i) {
         if (!fOpLists[i]->instantiate(fContext->resourceProvider())) {
+            SkDebugf("OpList failed to instantiate.\n");
             fOpLists[i] = nullptr;
             continue;
         }