TF will instantiate the Assembler when we're already on a background
thread, so it's not safe to read out the heap's max_old_generation_size
(it can change). This CL simply removes the use of that value from the
assembler. If the buffer gets too large we will fail when creating the
actual code object.

Bug: v8:6048
Change-Id: Ifb8a64c90222e4516117d237b001779fae060d28
Reviewed-on: https://chromium-review.googlesource.com/567921Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46581}