re-land of switched skslc from std::string to SkString

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5084

Change-Id: Ib21c30afc0d8483392b417e660b7fecfcc30e617
Reviewed-on: https://skia-review.googlesource.com/5084
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/sksl/ir/SkSLFunctionDefinition.h b/src/sksl/ir/SkSLFunctionDefinition.h
index ace27a3..bae8825 100644
--- a/src/sksl/ir/SkSLFunctionDefinition.h
+++ b/src/sksl/ir/SkSLFunctionDefinition.h
@@ -24,7 +24,7 @@
     , fDeclaration(declaration)
     , fBody(std::move(body)) {}
 
-    std::string description() const override {
+    SkString description() const override {
         return fDeclaration.description() + " " + fBody->description();
     }