Migrate ax::TextEncoder and decoder to gin::Wrappable

gin::Wrappable gets refactored to improve security and memory
management. This CL migrates ax::TextEncoder and ax::TextDecoder from
the old gin::Wrappable to the new gin::Wrappable.

Bug: 345640553
Change-Id: I9bba82c124522d990b1dd70ae96211c5d3a95dea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6722236
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1485744}