Decouple the --thin-strings from the --turbo flag.

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}

src/flag-definitions.h

@@ -270,7 +270,6 @@ DEFINE_BOOL(future, FUTURE_BOOL,
 DEFINE_IMPLICATION(future, turbo)
 
 DEFINE_IMPLICATION(turbo, ignition)
-DEFINE_IMPLICATION(turbo, thin_strings)
 
 // Flags for experimental implementation features.
 DEFINE_BOOL(allocation_site_pretenuring, true,
@@ -940,7 +939,7 @@ DEFINE_BOOL(native_code_counters, false,
             "generate extra code for manipulating stats counters")
 
 // objects.cc
-DEFINE_BOOL(thin_strings, false, "Enable ThinString support")
+DEFINE_BOOL(thin_strings, true, "Enable ThinString support")
 DEFINE_BOOL(trace_weak_arrays, false, "Trace WeakFixedArray usage")
 DEFINE_BOOL(trace_prototype_users, false,
             "Trace updates to prototype user tracking")