Add Drawing Manager guards against re-entrant flushes

src/gpu/GrDrawingManager.h

Index: src/gpu/GrDrawingManager.h
diff --git a/src/gpu/GrDrawingManager.h b/src/gpu/GrDrawingManager.h
index 672f7b08da71fae3b15b7856144bc810e834e24e..c72dad976bd184b7b88c48a143d8c40ef5c96507 100644
--- a/src/gpu/GrDrawingManager.h
+++ b/src/gpu/GrDrawingManager.h
@@ -60,7 +60,8 @@ private:
         , fNVPRTextContext(nullptr)
         , fPathRendererChain(nullptr)
         , fSoftwarePathRenderer(nullptr)
-        , fFlushState(context->getGpu(), context->resourceProvider()) {
+        , fFlushState(context->getGpu(), context->resourceProvider())
+        , fFlushing(false) {
         sk_bzero(fTextContexts, sizeof(fTextContexts));
     }
 
@@ -87,6 +88,7 @@ private:
     GrSoftwarePathRenderer*     fSoftwarePathRenderer;
 
     GrBatchFlushState           fFlushState;
+    bool                        fFlushing;
 };
 
 #endif