Add assert that memory pool is empty when GrTextBlobCache frees

BUG=skia:
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Review URL: https://codereview.chromium.org/1555993005
diff --git a/src/gpu/text/GrTextBlobCache.cpp b/src/gpu/text/GrTextBlobCache.cpp
index f11b7c6..685284b 100644
--- a/src/gpu/text/GrTextBlobCache.cpp
+++ b/src/gpu/text/GrTextBlobCache.cpp
@@ -55,4 +55,7 @@
         ++iter;
     }
     fCache.rewind();
+
+    // There should be no allocations in the memory pool at this point
+    SkASSERT(fPool.isEmpty());
 }