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/ast/SkSLASTIdentifier.h b/src/sksl/ast/SkSLASTIdentifier.h
index d67f64d..aa0179a 100644
--- a/src/sksl/ast/SkSLASTIdentifier.h
+++ b/src/sksl/ast/SkSLASTIdentifier.h
@@ -16,15 +16,15 @@
  * An identifier in an expression context. 
  */
 struct ASTIdentifier : public ASTExpression {
-    ASTIdentifier(Position position, std::string text)
+    ASTIdentifier(Position position, SkString text)
     : INHERITED(position, kIdentifier_Kind)
     , fText(std::move(text)) {}
 
-    std::string description() const override {
+    SkString description() const override {
         return fText;
     }
 
-    const std::string fText;
+    const SkString fText;
 
     typedef ASTExpression INHERITED;
 };