Revert "Disable --string-slices."

This reverts commit fd5b067f.

Reason for revert: Tanks Babylon by around 40%

Original change's description:
> 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}

TBR=yangguo@chromium.org,bmeurer@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7025
Change-Id: I95dc36e632ecb5ddcddda8f6f58528439d5c102b
Reviewed-on: https://chromium-review.googlesource.com/753621Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49116}

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, false, "use string slices")
+DEFINE_BOOL_READONLY(string_slices, true, "use string slices")
 
 // Flags for Ignition.
 DEFINE_BOOL(ignition_elide_noneffectful_bytecodes, true,