[desc] Add runtime_deps handler

Runtime deps are not shown by default in the desc command and thus were
missed in the recent refactoring. Adding them in.

Bug: crbug.com/gn/62

Change-Id: I80d374545425b886960436acf079a7a485e841b6
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/4460
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Julie Hockett <juliehockett@google.com>

tools/gn/command_desc.cc

diff --git a/tools/gn/command_desc.cc b/tools/gn/command_desc.cc
index b8b5355..a499384 100644
--- a/tools/gn/command_desc.cc
+++ b/tools/gn/command_desc.cc
@@ -291,7 +291,8 @@
           {variables::kDataKeys, DefaultHandler},
           {variables::kRebase, DefaultHandler},
           {variables::kWalkKeys, DefaultHandler},
-          {variables::kWriteOutputConversion, DefaultHandler}};
+          {variables::kWriteOutputConversion, DefaultHandler},
+          {"runtime_deps", DefaultHandler}};
 }
 
 void HandleProperty(const std::string& what,