Hex encode BinaryUploadService tokens

Tokens are passed as string fields in a proto, and therefore should
probably be valid UTF-8, but RandBytes won't generate valid UTF-8.
There's no immediate concerns here, but we don't want to run into
trouble if some system tries to validate the token field.

Bug: 980777
Change-Id: Iefe3391eff6325cfea732bc9004bb495754935d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814694
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Bettina Dea <bdea@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698612}

chrome/browser/safe_browsing/download_protection/binary_upload_service.cc

@@ -61,6 +61,7 @@ void BinaryUploadService::UploadForDeepScanning(
   }
 
   std::string token = base::RandBytesAsString(128);
+  token = base::HexEncode(token.data(), token.size());
   active_tokens_[raw_request] = token;
   binary_fcm_service_->SetCallbackForToken(
       token, base::BindRepeating(&BinaryUploadService::OnGetResponse,