When I tried to set the building target(workstation.arm64), I got this error:
ERROR Unresolved dependencies.
//:additional_cache_packages(//build/toolchain/fuchsia:arm64)
needs //src/chromium:chrome_browser(//build/toolchain/fuchsia:arm64)
How can I add chromium:chrome_browser to fx?
Dependencies like this are resolved automatically by the GN build system. The error you are seeing is because GN cannot find the package required by that product in the source tree.
More specifically, the workstation product currently depends on the //src/chromium:chrome_browser package targets, which are currently only available in the build graph for x64 targets. You can verify this by checking if you can successfully call fx set workstation.x64.
Regarding workstation support on arm64, I would recommend filing an issue so the team can determine if support can be added to the build or if the supported CPUs for workstation need to be documented.