On supported systems (as of this writing only 64-bit Linux) the appropriate architecture-specific subdirectory will contain a file named mojom after gclient sync is executed. The file mojom is an executable built from (some version of) the Go code in https://github.com/domokit/mojo/tree/master/mojom/mojom_tool.
The file mojom is downloaded by gclient sync from Google Cloud Storage from a file named mojo/mojom_tool/arch/sha1 where arch is an architecture-specific folder name and sha1 is the SHA1 digest of the file. Each architecture-specific subdirectory contains a file named mojom.sha1, for example linux64/mojom.sha1, that specifies the SHA1 digest of the current version of the binary on that architecture and lets gclient sync decide whether or not the binary is already up-to-date.
To browse the Google Cloud Storage bucket go to https://console.developers.google.com/storage/browser/mojo/mojom_parser/.
To update the version of mojom that will be downloaded by gclient sync, see https://github.com/domokit/mojo/blob/master/mojom/tools/build_mojom_tool.py.