Add assert that memory pool is empty when GrTextBlobCache frees

src/gpu/text/GrTextBlobCache.cpp

Index: src/gpu/text/GrTextBlobCache.cpp
diff --git a/src/gpu/text/GrTextBlobCache.cpp b/src/gpu/text/GrTextBlobCache.cpp
index f11b7c60cbd88d62857863e27847d9132c8219a8..685284bee5c02fd062bb84ab29b4ed85feabf8b9 100644
--- a/src/gpu/text/GrTextBlobCache.cpp
+++ b/src/gpu/text/GrTextBlobCache.cpp
@@ -55,4 +55,7 @@ void GrTextBlobCache::freeAll() {
         ++iter;
     }
     fCache.rewind();
+
+    // There should be no allocations in the memory pool at this point
+    SkASSERT(fPool.isEmpty());
 }