Add gdb macro for printing v8::Local content.

tools/gdbinit

Index: tools/gdbinit
diff --git a/tools/gdbinit b/tools/gdbinit
index cdf2cd4e222a762c645504a89f2038e1a2fa6287..3d527829bf85b92b62db79a98b7a0b369c2c8bf8 100644
--- a/tools/gdbinit
+++ b/tools/gdbinit
@@ -11,6 +11,15 @@ 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))