GitLab

Creates a decoder buffer base class that exposes only the properties
of a decoder buffer. This makes it possible to hide how a buffer
is created and how it is organized in memory, this is left
as an implementation detail of derived classes.

BUG=408189

Review URL: https://codereview.chromium.org/535563002

Cr-Commit-Position: refs/heads/master@{#293298}