Calling AstNode::Print() from within gdb, e.g. during bytecode
generation, sometimes results in a CHECK failure due to handle
dereferencing being disallowed.  This is very frustrating.

Since this function is purely for debugging and not called anywhere in
the code base, I'm now explicitly adding an AllowHandleDereference scope
to it.

R=adamk@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2711003007
Cr-Commit-Position: refs/heads/master@{#43439}