Some compilers complain that ConvertReturn is private in SimulatorBase,
but used in the arm64 Simulator. This CL fixes this by making
ConvertReturn protected.

R=ulan@chromium.org

Bug: v8:7541
Change-Id: I9326b2c746829e9c37c58c14561811b32929c615
Reviewed-on: https://chromium-review.googlesource.com/955689Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51834}