ENH: pass cache_dir arg through fetch, webrtc, and eventually chromium.

BUG=

Change-Id: I33aa83c36721059005b86fe8538560c302bbd4b5
Reviewed-on: https://chromium-review.googlesource.com/414692
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Henrik Kjellander <kjellander@chromium.org>

fetch.py

@@ -114,7 +114,7 @@ class GclientGitCheckout(GclientCheckout, GitCheckout):
                              for key, value in soln.iteritems())
       soln_strings.append('  {\n%s\n  },' % soln_string)
     gclient_spec = 'solutions = [\n%s\n]\n' % '\n'.join(soln_strings)
-    extra_keys = ['target_os', 'target_os_only']
+    extra_keys = ['target_os', 'target_os_only', 'cache_dir']
     gclient_spec += ''.join('%s = %s\n' % (key, _format_literal(self.spec[key]))
                              for key in extra_keys if key in self.spec)
     return gclient_spec

fetch_configs/webrtc.py

@@ -31,6 +31,9 @@ class WebRTC(config_util.Config):
     if props.get('target_os'):
       spec['target_os'] = props['target_os'].split(',')
 
+    if props.get('cache_dir'):
+      spec['cache_dir'] = props['cache_dir']
+
     return {
       'type': 'gclient_git',
       'gclient_git_spec': spec,