GN Templates: Move common_config to the template.

Remove common_config from the targets' config and add
it to the template instead.

BUG=webrtc:6187
NOTRY=True

Review-Url: https://codereview.webrtc.org/2300413002
Cr-Original-Commit-Position: refs/heads/master@{#14063}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7a2ce0b73817bfe934062c207bfa638d23c6b809