Add assert that memory pool is empty when GrTextBlobCache frees

src/gpu/GrDrawingManager.cpp

Index: src/gpu/GrDrawingManager.cpp
diff --git a/src/gpu/GrDrawingManager.cpp b/src/gpu/GrDrawingManager.cpp
index 7aa0fdbe0a281094edff5936373012cc03353c91..2a27f6aac453557aca310f1629563d01929f6348 100644
--- a/src/gpu/GrDrawingManager.cpp
+++ b/src/gpu/GrDrawingManager.cpp
@@ -20,6 +20,9 @@ void GrDrawingManager::cleanup() {
         fDrawTargets[i]->makeClosed();  // no drawTarget should receive a new command after this
         fDrawTargets[i]->clearRT();
 
+        // We shouldn't need to do this, but it turns out some clients still hold onto drawtargets
+        // after a cleanup
+        fDrawTargets[i]->reset();
         fDrawTargets[i]->unref();
     }