[WebSocket] Merge duplicated HTTP headers for devtools

The new WebSocket implementation receives HTTP headers on
the opening handshake from the browser process.
The headers may be duplicated, e.g. there are two "Foo" headers.
In order to show duplicated headers in devtools, this CL merges them
during storing them to HTTPHeaderMap.
This CL changes the existing WebSocket behavior as well.

R=tyoshino
BUG=310405

Review URL: https://codereview.chromium.org/130863002

git-svn-id: svn://svn.chromium.org/blink/trunk@165044 bbb929c8-8fbe-4397-9dbb-9b2b20218538