diagnostics: Constant for Mojo pipe token Add a constant with the Mojo pipe token for the Mojo pipe between the browser and the diagnosticsd daemon. This constant needs to be known on both ends of this Mojo pipe. For example, on the browser side, that's the name that is passed to mojo::OutgoingInvitation::AttachMessagePipe(). BUG=chromium:869377 TEST=system_api package builds Change-Id: I3020c07de5493dac0740e43a4ec2d1791105e805 Reviewed-on: https://chromium-review.googlesource.com/1230873 Commit-Ready: Maksim Ivanov <[email protected]> Tested-by: Maksim Ivanov <[email protected]> Reviewed-by: Roman Sorokin <[email protected]>