On Windows, depot tools will download a special version of Git during your first gclient sync. On Mac and Linux, you'll need to install Git by yourself.
A script is provided for Ubuntu, which is unfortunately only available after your first gclient sync:
$ ./build/install-build-deps.sh
Most of the libraries installed with this script are not needed since we now build using Debian sysroot images in build/linux, but there are still some tools needed for the build that are installed with install-build-deps.sh.
You may also want to have a look at the Chromium Linux Build instructions if you experience any other problems building.
Follow the Chromium's build instructions for Windows.
WebRTC requires Visual Studio 2017 to be used. If you only have version 2015 available, you might be able to keep using it for some time by setting GYP_MSVS_VERSION=2015 in your environment. Keep in mind that this is not a suppported configuration however.
Xcode 12 or higher is required. Latest Xcode is recommended to be able to build all code. You may use xcode-select --install to install it.
Absence of Xcode will cause errors like:
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
You‘ll need a Linux development machine. WebRTC is using the same Android toolchain as Chrome (downloaded into third_party/android_tools) so you won’t need to install the NDK/SDK separately.