[CrOS PhoneHub] Log payload size instead of content.

Ensure we don't accidentally log PII by only logging the size.

Bug: 1106937
Change-Id: I428ac50ea33b6bc5780fcf0fc47ca92cb151a612
Fixed: 1143033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522190
Commit-Queue: Andre Le <leandre@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824596}

chromeos/services/secure_channel/secure_channel.cc

@@ -284,7 +284,7 @@ void SecureChannel::OnMessageDecoded(const std::string& feature,
   PA_LOG(VERBOSE) << "Received message from " << connection_->GetDeviceAddress()
                   << ": {"
                   << "feature: \"" << feature << "\", "
-                  << "payload: \"" << decoded_message << "\""
+                  << "payload size: " << decoded_message.size() << " byte(s)"
                   << "}";
 
   for (auto& observer : observer_list_)