Both TurboFan and ThinStrings have been enabled by default for a while.
This just disentangles the implication between those two flags and sets
the --thin-strings individually. There is no technical reason for the
implication.

R=jkummerow@chromium.org

Change-Id: I26e5357ffaf953de897c76d6edb8ac640bbeafd0
Reviewed-on: https://chromium-review.googlesource.com/528076Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45789}