Fix Scanner invariants w/ literal buffers.

This isn't the most elegant fix, but I'd prefer to not rework the logic
right now. What happens is:
- Most parts of the Scanner use nullptr to mean, no literal buffer.
- The bookmarking logic may end up with a state where there's a non-nullptr
  literal buffer, but it's empty. (length 0)
- These are functionally equivalent, so there's no 'real' bug.
- But it makes it hard to reason. This patch hence checks for length-0
  literal buffers, and uses nullptr instead.

R=marja@chromium.org
BUG=chromium:639191 v8:4947

Review-Url: https://codereview.chromium.org/2258073003
Cr-Commit-Position: refs/heads/master@{#38756}

src/parsing/scanner.cc

@@ -1598,14 +1598,9 @@ void Scanner::ResetToBookmark() {
 
   source_->ResetToBookmark();
   c0_ = bookmark_c0_;
-  StartLiteral();
-  StartRawLiteral();
-  CopyTokenDesc(&next_, &bookmark_current_);
+  CopyToNextTokenDesc(&bookmark_current_);
   current_ = next_;
-  StartLiteral();
-  StartRawLiteral();
-  CopyTokenDesc(&next_, &bookmark_next_);
-
+  CopyToNextTokenDesc(&bookmark_next_);
   bookmark_c0_ = kBookmarkWasApplied;
 }
 
@@ -1620,6 +1615,13 @@ bool Scanner::BookmarkHasBeenReset() {
 
 void Scanner::DropBookmark() { bookmark_c0_ = kNoBookmark; }
 
+void Scanner::CopyToNextTokenDesc(TokenDesc* from) {
+  StartLiteral();
+  StartRawLiteral();
+  CopyTokenDesc(&next_, from);
+  if (next_.literal_chars->length() == 0) next_.literal_chars = nullptr;
+  if (next_.raw_literal_chars->length() == 0) next_.raw_literal_chars = nullptr;
+}
 
 void Scanner::CopyTokenDesc(TokenDesc* to, TokenDesc* from) {
   DCHECK_NOT_NULL(to);

src/parsing/scanner.h

@@ -534,6 +534,7 @@ class Scanner {
   bool BookmarkHasBeenSet();
   bool BookmarkHasBeenReset();
   void DropBookmark();
+  void CopyToNextTokenDesc(TokenDesc* from);
   static void CopyTokenDesc(TokenDesc* to, TokenDesc* from);
 
   void ReportScannerError(const Location& location,