Remoting: Add missing Gyp dependency on the "prep_toolchain" target

remoting/remoting_key_tester.gypi

Index: remoting/remoting_key_tester.gypi
diff --git a/remoting/remoting_key_tester.gypi b/remoting/remoting_key_tester.gypi
index ca8c5029a621ffee6aedac8c6a131c1597c21b28..1d9e766d2d0098713bef00e72c4224d3a97d3d50 100644
--- a/remoting/remoting_key_tester.gypi
+++ b/remoting/remoting_key_tester.gypi
@@ -95,6 +95,10 @@
             '-lppapi_stub',
             '-lppapi_cpp',
           ],
+          'dependencies': [
+            # TODO(mseaborn): Remove need for this (https://crbug.com/456902).

[Łukasz Anforowicz, 2015/02/09 20:57:07]

Mark, do you also want/need a similar TODO comment in remoting/remoting_nacl.gyp
(which also lists prep_toolchain as a dependency)?

BTW: My apologies for missing this dependency.  Are there some pre-check-in
steps that I should have done (that would have caught the missing dependency
before landing crrev.com/884703006)?

[Mark Seaborn, 2015/02/17 22:56:35]

I thought about doing that, but since ncbray is actively working on removing the
need for this (https://crbug.com/456902), I didn't think it would be worth
adding comments to other locations.  Hopefully the fix will be committed in
Chromium by tomorrow.
No worries.  We were only catching this by manual review before, but ncbray's
cleanup will remove the need for that.

+            '<(DEPTH)/native_client/tools.gyp:prep_toolchain',
+          ],
         },  # end of target 'remoting_key_tester_pexe'
       ],
     }]