Struct fields should not end in an underscore according to the style
guide.

Drive-by: Add {TurboAssembler} constructor which receives an
{IsolateData} directly, to allow creating a {TurboAssembler} in a
background thread.

R=mstarzinger@chromium.org

Change-Id: I32800476690f4c8619059519b7d27b06f5d4be95
Reviewed-on: https://chromium-review.googlesource.com/1090278
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53582}