Add gdb macro for printing v8::Local content.

R=marja@chromium.org

Review-Url: https://codereview.chromium.org/2628293003
Cr-Commit-Position: refs/heads/master@{#42327}

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))