[Extensions] Remove linked_ptr usage from extension_messages.cc

This CL also makes ExtensionMsg_Load_Params non-copyable. There
shouldn't be any behavior change in this CL.

Bug: 556939
Change-Id: If7d11919f8fee0442578371d44d6b715d08efe3a
Reviewed-on: https://chromium-review.googlesource.com/1132572
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574606}
4 files changed