Add a class to manage SbPlayer.

This is part of the work to implement a Chromium renderer via Starboard.

Test: Added unit tests

BYPASS_LARGE_CHANGE_WARNING
http://go/small-cls specifically states "the
right size for a CL is one self-contained change" This CL adds one
single class and unit tests for it.

Bug: b:323610278
Change-Id: Icafa37cc79dd21632a7b0a3d3f3a8c66b4de35b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6615537
Reviewed-by: Shawn Quereshi <shawnq@google.com>
Commit-Queue: Antonio Rivera <antoniori@google.com>
Cr-Commit-Position: refs/heads/main@{#1470287}