Add assert that memory pool is empty when GrTextBlobCache frees

src/gpu/text/GrTextBlobCache.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrTextBlobCache.h"
 
 static const int kVerticesPerGlyph = 4;
@@ skipping 37 matching lines @@
 
 void GrTextBlobCache::freeAll() {
     SkTDynamicHash<GrAtlasTextBlob, GrAtlasTextBlob::Key>::Iter iter(&fCache);
     while (!iter.done()) {
         GrAtlasTextBlob* blob = &(*iter);
         fBlobList.remove(blob);
         blob->unref();
         ++iter;
     }
     fCache.rewind();
+
+    // There should be no allocations in the memory pool at this point
+    SkASSERT(fPool.isEmpty());
 }