Restructure protobuf targets to fit with downstream requirements.

The paths of the protobuf output files needs to match the actual tree path
in order for Bazel builds to work.

BUG=webrtc:6412
NOTRY=True

Review-Url: https://codereview.webrtc.org/2716413004
Cr-Original-Commit-Position: refs/heads/master@{#16912}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e3e902eef9dcf94342336cd982076fe69df2db4b
4 files changed