[Extensions] Use type-safe functions to fill the replies

When using delayed replies for IPC messages, we can hold the
IPC::Message* with the reply and fill it with values later, before
sending.

We used the more general function IPC::WriteParam, but one drawback is
that it doesn't do any type checking. Message macros / templates already
generate proper functions that will do the necessarily checking, so we
can call the appropriate WriteReplyParams() of the message we are
replying.

In XWalkExtensionServer because ListValue doesn't have a copy
constructor, we construct the typed reply param ourselves and write
it. It is more verbose but also type-safe.
Merge pull request #1015 from cmarcelo/better-reply-write

[Extensions] Use type-safe functions to fill the replies