Remove libwebm third_party dependency.

libwebm was used in remoting, but is not needed anymore.

Committed: https://crrev.com/c053fb4fec709c4a5a72065305ee4eb2da2efaa8
Cr-Commit-Position: refs/heads/master@{#309243}

Review URL: https://codereview.chromium.org/820593002

Cr-Commit-Position: refs/heads/master@{#309295}
12 files changed