Allow empty header value in the blink HTTP header parser.

The HTTP header parser in Blink forbids empty header values which should
be allowed. This CL fix the issue.

BUG=380075
R=tyoshino@chromium.org

Review URL: https://codereview.chromium.org/318613002

git-svn-id: svn://svn.chromium.org/blink/trunk@175532 bbb929c8-8fbe-4397-9dbb-9b2b20218538