this issue is introduced by 30787d4e.
   (https://codereview.chromium.org/874693006).

   original commit message:

    Const-corrected some MachineSignature* parameters.

    Recipe: Add "const" to RawMachineAssembler constructor, fix breakage.

BUG=

Review URL: https://codereview.chromium.org/905283002

Cr-Commit-Position: refs/heads/master@{#26538}