Also consider merging https://chromium.googlesource.com/chromium/blink/+/085f6a22460b323408b9794f17f93cef647ff898. Profiling memory use by Mobile Gmail shows that we're wasting a lot of memory on duplicate HTTP headers. For example, we're wasting 2.4 kB by having 25 copies of the User-Agent header. To save memory, we should atomize header values as well as header names.