This completes the transition to Assembler::Options, which reduces
the assemblers's dependency on isolates, and there is now only one
way to create an Assembler, which is to use the options.
Note that some operations on assemblers still need an isolate, such
as GetCode(), and in these cases, the isolate is an additional
argument to the method.

R=jgruber@chromium.org
CC=mstarzinger@chromium.org

Change-Id: I413209d816c63a7c3640f1c226764693dcad1e7f
Reviewed-on: https://chromium-review.googlesource.com/1106169
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53925}