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/SkSLField.h b/src/sksl/ir/SkSLField.h
index a01df29..f73b602 100644
--- a/src/sksl/ir/SkSLField.h
+++ b/src/sksl/ir/SkSLField.h
@@ -26,7 +26,7 @@
     , fOwner(owner)
     , fFieldIndex(fieldIndex) {}
 
-    virtual std::string description() const override {
+    virtual SkString description() const override {
         return fOwner.description() + "." + fOwner.fType.fields()[fFieldIndex].fName;
     }