This change does not change how the BufferedRawMachineAssemblerTester is
used in tests, but it makes its construction in other constructors (e.g.
in the WasmRunner) cleaner.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1414203009

Cr-Commit-Position: refs/heads/master@{#31850}