Don't purge resources for trivial GrContext flushes

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 5f4e663580a7e525cdac0f07f110a9b7f70e909e..6221fea3c66fa112243967a2efc0f3a6966e55de 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -228,13 +228,15 @@ void GrContext::TextBlobCacheOverBudgetCB(void* data) {
 void GrContext::flush(int flagsBitfield) {
     ASSERT_SINGLE_OWNER
     RETURN_IF_ABANDONED
-
+    bool flushed = false;
     if (kDiscard_FlushBit & flagsBitfield) {
         fDrawingManager->reset();
     } else {
-        fDrawingManager->flush();
+        flushed = fDrawingManager->flush();
+    }
+    if (flushed) {
+        fResourceCache->notifyFlushOccurred();
     }
-    fResourceCache->notifyFlushOccurred();
     fFlushToReduceCacheSize = false;
 }