Add builtin flag to SymbolTable.

This flag is set on anything rehydrated and on the root symbol table;
that is, anything that will outlive your Program's tree.

Change-Id: Idd9a167ee69f1fb9e526aecbee733ce1ccb5d265
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327920
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/sksl/SkSLDehydrator.cpp b/src/sksl/SkSLDehydrator.cpp
index 374166e..1a6c4a9 100644
--- a/src/sksl/SkSLDehydrator.cpp
+++ b/src/sksl/SkSLDehydrator.cpp
@@ -436,7 +436,7 @@
                 this->write(f.test().get());
                 this->write(f.next().get());
                 this->write(f.statement().get());
-                this->write(f.symbols());
+                this->write(*f.symbols());
                 break;
             }
             case Statement::Kind::kIf: {