Add additional assert to GrSingleOwner

TBR=bsalomon@google.com
TBR=mtklein@google.com
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1566703003

Review URL: https://codereview.chromium.org/1566703003

include/private/GrSingleOwner.h

@@ -37,6 +37,7 @@ private:
 
      void exit() {
          SkAutoMutexAcquire lock(fMutex);
+         SkASSERT(fOwner == SkGetThreadID());
          fReentranceCount--;
          if (fReentranceCount == 0) {
              fOwner = kIllegalThreadID;