[Sync] Do not use base::RandBytesAsString to generate random string

components/sync/engine_impl/commit.cc

Index: components/sync/engine_impl/commit.cc
diff --git a/components/sync/engine_impl/commit.cc b/components/sync/engine_impl/commit.cc
index 877ba4ca07983cf5524449ee07669a1ca2a81059..2c9485080023fe7bac84f37a608c42608cc13369 100644
--- a/components/sync/engine_impl/commit.cc
+++ b/components/sync/engine_impl/commit.cc
@@ -27,8 +27,19 @@ namespace {
 // because it is not too large (to hurt performance and compression ratio), but
 // it is not too small to easily be canceled out using statistical analysis.
 const size_t kPaddingSize = 256;
+
+std::string RandASCIIString(size_t length) {
+  std::string result;
+  const int kMin = static_cast<int>(' ');
+  const int kMax = static_cast<int>('~');
+  result.reserve(length);
+  for (size_t i = 0; i < length; ++i)
+    result.push_back(static_cast<char>(base::RandInt(kMin, kMax)));
+  return result;
 }
 
+}  // namespace
+
 Commit::Commit(ContributionMap contributions,
                const sync_pb::ClientToServerMessage& message,
                ExtensionsActivity::Records extensions_activity_buffer)
@@ -69,7 +80,7 @@ Commit* Commit::Init(ModelTypeSet requested_types,
 
   // Set padding to mitigate CRIME attack.
   if (base::FeatureList::IsEnabled(syncer::kSyncClientToServerCompression)) {
-    commit_message->set_padding(base::RandBytesAsString(kPaddingSize));
+    commit_message->set_padding(RandASCIIString(kPaddingSize));
   }
 
   // Set extensions activity if bookmark commits are present.