The following is a short explanation of why we use a the hermetic toolchain and instructions on how to roll a new toolchain.
Download a new version of Xcode, and confirm either mac or ios builds properly with this new version.
Run the following command:
src/build/package_mac_toolchain.py /path/to/Xcode.app/ [ios|mac]
The script will create a subset of the toolchain necessary for a build, and upload them to be used by hermetic builds.
If for some reason this toolchain version has already been uploaded, the script will ask if we should create sub revision. This can be necessary when the package script has been updated to compress additional files.
Create a CL with updated [MAC|IOS]_TOOLCHAIN_VERSION and _SUB_REVISION in src/build/mac_toolchain.py with the version created by the previous command.
Run the CL through the trybots to confirm the roll works.
Building Chrome Mac currently requires many binaries that come bundled with Xcode, as well the macOS and iphoneOS SDK [also bundled with Xcode]. Note that Chrome ships its own version of clang [compiler], but is dependent on Xcode for these other binaries.
Chrome should be built against the latest SDK available, but historically, updating the SDK has been nontrivially difficult. Additionally, bot system installs can range from Xcode 5 on some bots, to the latest and greatest. Using a hermetic toolchain has two main benefits:
Build Chrome with a well-defined toolchain [rather than whatever happens to be installed on the machine].
Easily roll/update the toolchain.