Fix uninitialized member (isolate_) in AstPrinter.

src/ast/prettyprinter.cc

Index: src/ast/prettyprinter.cc
diff --git a/src/ast/prettyprinter.cc b/src/ast/prettyprinter.cc
index b94f0951cead9af488c5b2be0ee47a0a09c10e81..2e153cf059dabfb8645f579d360df99bd27ca162 100644
--- a/src/ast/prettyprinter.cc
+++ b/src/ast/prettyprinter.cc
@@ -596,7 +596,7 @@ class IndentedScope BASE_EMBEDDED {
 //-----------------------------------------------------------------------------
 
 AstPrinter::AstPrinter(Isolate* isolate)
-    : output_(nullptr), size_(0), pos_(0), indent_(0) {
+    : isolate_(isolate), output_(nullptr), size_(0), pos_(0), indent_(0) {
   InitializeAstVisitor(isolate);
 }