Fix UnindexedRulesetWriter finishing.

components/subresource_filter/core/common/unindexed_ruleset_unittest.cc

Index: components/subresource_filter/core/common/unindexed_ruleset_unittest.cc
diff --git a/components/subresource_filter/core/common/unindexed_ruleset_unittest.cc b/components/subresource_filter/core/common/unindexed_ruleset_unittest.cc
index 5c093b3aa25609c902a6cff839192c75ec88bd3c..812d1b9ea097c9c1a9c30a9164c14e69cb53d0a2 100644
--- a/components/subresource_filter/core/common/unindexed_ruleset_unittest.cc
+++ b/components/subresource_filter/core/common/unindexed_ruleset_unittest.cc
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/numerics/safe_conversions.h"
 #include "base/strings/string_number_conversions.h"
 #include "components/subresource_filter/core/common/proto/rules.pb.h"
 #include "components/subresource_filter/core/common/url_pattern.h"
@@ -122,6 +123,10 @@ bool IsRulesetValid(const std::string& ruleset_contents,
     read_rules.insert(read_rules.end(), chunk.url_rules().begin(),
                       chunk.url_rules().end());
   }
+  if (base::checked_cast<size_t>(reader.num_bytes_read()) !=
+      ruleset_contents.size()) {
+    return false;
+  }
 
   if (expected_url_rules.size() != read_rules.size())
     return false;