Make DependencyGraph.DeepDependencies() depth-first.

test/dependencies/adso/all_dependent_settings_order.gyp

+{
+  'targets': [
+    {
+      'target_name': 'a',
+      'type': 'none',
+      'sources': ['a.cc'],
+      'all_dependent_settings': {'sources': ['a.cc']},

[Nico, 2015/06/18 21:51:58]

Do direct_dependent_settings work correctly already?

[Dan Beam, 2015/06/18 21:56:27]

unless I'm misunderstanding something: they might but they don't use
DeepDependencies() nor recursion[1].  I can write another test for them if you'd
like, but it seems like a separate issue.

[1]
https://code.google.com/p/chromium/codesearch#chromium/src/tools/gyp/pylib/gy...

+    },
+    {
+      'target_name': 'b',
+      'type': 'none',
+      'sources': ['b.cc'],
+      'all_dependent_settings': {'sources': ['b.cc']},
+      'dependencies': ['a'],
+    },
+
+    {
+      'target_name': 'c',
+      'type': 'none',
+      'sources': ['c.cc'],
+      'all_dependent_settings': {'sources': ['c.cc']},
+      'dependencies': ['b', 'a'],
+    },
+    {
+      'target_name': 'd',
+      'type': 'none',
+      'sources': ['d.cc'],
+      'dependencies': ['c', 'a', 'b'],
+      'actions': [
+        {
+          'action_name': 'write_sources',
+          'inputs': ['write_args.py'],
+          'outputs': ['<(PRODUCT_DIR)/out.txt'],
+          'action': [
+            'python',
+            'write_args.py',
+            '<(PRODUCT_DIR)/out.txt',
+            '>@(_sources)'
+          ],
+          'msvs_cygwin_shell': 0,
+        },
+      ],
+    },
+  ],
+}