[printing] Print properties backing store value and add a gdb macro for printing LayoutDescriptors.

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
@@ skipping 20 matching lines @@
 
 # Print DescriptorArray.
 define jda
 call _v8_internal_Print_DescriptorArray((void*)($arg0))
 end
 document jda
 Print a v8 DescriptorArray object
 Usage: jda tagged_ptr
 end
 
+# Print LayoutDescriptor.
+define jld
+call _v8_internal_Print_LayoutDescriptor((void*)($arg0))
+end
+document jld
+Print a v8 LayoutDescriptor object
+Usage: jld tagged_ptr
+end
+
 # Print TransitionArray.
 define jta
 call _v8_internal_Print_TransitionArray((void*)($arg0))
 end
 document jta
 Print a v8 TransitionArray object
 Usage: jta tagged_ptr
 end
 
 # Print JavaScript stack trace.
@@ skipping 39 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