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>
diff --git a/src/sksl/ir/SkSLConstructor.h b/src/sksl/ir/SkSLConstructor.h
index 0501b65..92f7580 100644
--- a/src/sksl/ir/SkSLConstructor.h
+++ b/src/sksl/ir/SkSLConstructor.h
@@ -21,9 +21,9 @@
     : INHERITED(position, kConstructor_Kind, type)
     , fArguments(std::move(arguments)) {}
 
-    std::string description() const override {
-        std::string result = fType.description() + "(";
-        std::string separator = "";
+    SkString description() const override {
+        SkString result = fType.description() + "(";
+        SkString separator;
         for (size_t i = 0; i < fArguments.size(); i++) {
             result += separator;
             result += fArguments[i]->description();