media: Move key_system_names to media/base/

This file is already used from media/base/ and should be there to
clean up some dependency issues (as noted by gn check).

BUG=599578
TEST=compiles with gn and gyp

Review-Url: https://codereview.chromium.org/1921823003
Cr-Commit-Position: refs/heads/master@{#390276}
9 files changed