GitLab

Splits decoder logic into a seperate generated file, and breaks
logic into several templates with the same parameters.

Bug: 
Change-Id: I8d6bf188ec73a475428cc49991bdace36458a468
Reviewed-on: https://chromium-review.googlesource.com/721849Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509608}