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/SkSLVarDeclarations.h b/src/sksl/ir/SkSLVarDeclarations.h
index e64a874..295c0b6 100644
--- a/src/sksl/ir/SkSLVarDeclarations.h
+++ b/src/sksl/ir/SkSLVarDeclarations.h
@@ -27,8 +27,8 @@
     , fSizes(std::move(sizes))
     , fValue(std::move(value)) {}
 
-    std::string description() const {
-        std::string result = fVar->fName;
+    SkString description() const {
+        SkString result = fVar->fName;
         for (const auto& size : fSizes) {
             if (size) {
                 result += "[" + size->description() + "]";
@@ -57,13 +57,12 @@
     , fBaseType(*baseType)
     , fVars(std::move(vars)) {}
 
-    std::string description() const override {
+    SkString description() const override {
         if (!fVars.size()) {
-            return "";
+            return SkString();
         }
-        std::string result = fVars[0].fVar->fModifiers.description() + fBaseType.description() + 
-                             " ";
-        std::string separator = "";
+        SkString result = fVars[0].fVar->fModifiers.description() + fBaseType.description() + " ";
+        SkString separator;
         for (const auto& var : fVars) {
             result += separator;
             separator = ", ";