[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..b46d1c3a4dfcc928b6f543b6031a27bca8d753a2 100644
--- a/components/sync/engine_impl/commit.cc
+++ b/components/sync/engine_impl/commit.cc
@@ -27,8 +27,29 @@ 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;
+
+const char kLegalCharacters[] =
+    "abcdefghijklmnopqrstuvwxyz"
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+    "0123456789"
+    " .-,/\\:;";
+
+const int first_char_index = 0;
+const int last_char_index = sizeof(kLegalCharacters) - 2;
+
+std::string RandomString(int length) {

[maxbogue, 2017/01/20 01:24:56]

I was poking around trying to understand RandInt and sizeof and found this:

https://cs.chromium.org/chromium/src/components/history/core/browser/delete_d...

What do you think of copying that approach instead of hard-coding
kLegalCharacters? I think it's cleaner, though you should keep the
.reserve(length) call yours has.

[Gang Wu, 2017/01/20 07:46:56]

Done.

+  std::string random;
+  random.reserve(length);
+  for (int i = 0; i < length; ++i) {
+    random.push_back(
+        kLegalCharacters[base::RandInt(first_char_index, last_char_index)]);
+  }
+
+  return random;
 }
 
+}  // namespace
+
 Commit::Commit(ContributionMap contributions,
                const sync_pb::ClientToServerMessage& message,
                ExtensionsActivity::Records extensions_activity_buffer)
@@ -69,7 +90,7 @@ 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(RandomString(kPaddingSize));
   }
 
   // Set extensions activity if bookmark commits are present.