拉取webrtc源码并指定m128分支

1、安装depot_tools

git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
export PATH=$PATH:`pwd`/depot_tools

2、配置gclient

mkdir webrtc-checkout
cd webrtc-checkout
gclient config --name src https://webrtc.googlesource.com/src.git

3、同步源码

gclient sync --with_branch_heads --with_tags

4、切换到M128分支

cd src
git checkout branch-heads/6613