Add gdb macro for printing v8::Local content.

tools/gdbinit

 # Copyright 2014 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Print HeapObjects.
 define job
 call _v8_internal_Print_Object((void*)($arg0))
 end
 document job
 Print a v8 JavaScript object
 Usage: job tagged_ptr
 end
 
+# Print v8::Local handle value.
+define jlh
+call _v8_internal_Print_Object(*(v8::internal::Object**)(*$arg0))
+end
+document jlh
+Print content of a v8::Local handle
+Usage: jlh local_handle
+end
+
 # Print Code objects containing given PC.
 define jco
 call _v8_internal_Print_Code((void*)($arg0))
 end
 document jco
 Print a v8 Code object from an internal code address
 Usage: jco pc
 end
 
 # Print TypeFeedbackVector
@@ skipping 75 matching lines @@
     print("%s -> %s %s (%s)\033[0m" % (color, prefix, match.group(2), match.group(1)))
 end
 end
 document bta
 Print stack trace with assertion scopes
 Usage: bta
 end
 
 set disassembly-flavor intel
 set disable-randomization off