HTTPHeaderMap: Favor composition over inheritance.

dllexported classes export all methods of the base class on Windows.
By making the HashMap a member instead of a base, fewer methods get
instantiated.

No intended behavior change.

BUG=409105

Review URL: https://codereview.chromium.org/536523002

git-svn-id: svn://svn.chromium.org/blink/trunk@181266 bbb929c8-8fbe-4397-9dbb-9b2b20218538