Disable --string-slices.

This is an experiment to quantify the impact of SlicedStrings on both
performance and memory usage. The intention is to get Canary coverage
for the experiment and then decide how to proceed.

Bug: v8:7025
Change-Id: Ied548cd9e2fab127c1ad2aea3e60b2615d3de663
Reviewed-on: https://chromium-review.googlesource.com/750082
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49070}

src/flag-definitions.h

@@ -316,7 +316,7 @@ DEFINE_VALUE_IMPLICATION(optimize_for_size, max_semi_space_size, 1)
 
 // Flags for data representation optimizations
 DEFINE_BOOL(unbox_double_arrays, true, "automatically unbox arrays of doubles")
-DEFINE_BOOL_READONLY(string_slices, true, "use string slices")
+DEFINE_BOOL_READONLY(string_slices, false, "use string slices")
 
 // Flags for Ignition.
 DEFINE_BOOL(ignition_elide_noneffectful_bytecodes, true,