The component updater now only uses bsdiff so courgete diffs no longer have to forwards and backwards compatible, since Chrome always ships the correct version of the courgette patcher with the courgette diff.

However, bsdiff generated diffs still have to be version compatible.

This CL adds a test for the forwards compatible case.

BUG=459505

Review URL: https://codereview.chromium.org/918403003

Cr-Commit-Position: refs/heads/master@{#317117}