Revert "switched skslc from std::string to SkString"

Revert "switched skslc from std::string to SkString"

This reverts commit d8df21a1e08b5b3380261f4b90acfbdc538ef93c.

Reason for revert: Breaking Roll

Original change's description:
> switched skslc from std::string to SkString
>
> BUG=skia:
>
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4977
>
> Change-Id: I15e24963b09b719a2c07da67745114f5ac66cee8
> Reviewed-on: https://skia-review.googlesource.com/4977
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
>

TBR=bsalomon@google.com,benjaminwagner@google.com,ethannicholas@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I84d5311beb9d5e607b7a4a3c138332f0c8f19648
Reviewed-on: https://skia-review.googlesource.com/5077
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>