Add support for "additional_mirrors" property in trybot configs.

scripts/slave/recipe_modules/chromium_tests/trybots.py

Index: scripts/slave/recipe_modules/chromium_tests/trybots.py
diff --git a/scripts/slave/recipe_modules/chromium_tests/trybots.py b/scripts/slave/recipe_modules/chromium_tests/trybots.py
index 94ba4593e404ddfda9979eb98bec1349384bffa5..e0b00aa8827cfef8af222dad5dcc620ce2d22daa 100644
--- a/scripts/slave/recipe_modules/chromium_tests/trybots.py
+++ b/scripts/slave/recipe_modules/chromium_tests/trybots.py
@@ -267,6 +267,16 @@ TRYBOTS = freeze({
         'mastername': 'chromium.linux',
         'buildername': 'Linux Builder',
         'tester': 'Linux Tests',
+        # TODO(kbr): add this to the other trybots that run GPU tests.
+        # This is only being added now in order to achieve full code
+        # coverage in the recipes.
+        'additional_mirrors': [

[Sergey Berezin, 2016/01/06 01:50:56]

nit on the design: it might make sense to generalize the config, e.g. to:

  linux_chromium_rel_ng: [
   {
     'mastername': 'chromium.linux',
     'buildername': 'Linux Builder',
     'tester': 'Linux Tests',
   },
   {
      'mastername': 'chromium.gpu',
      'buildername': 'GPU Linux Builder',
      'tester': 'Linux Release (NVIDIA)',
    },
   ]

and allow a single dict as an option for backward compatibility, but interpreted
as a single element list. WDYT?

[Ken Russell (switch to Gerrit), 2016/01/06 03:30:07]

That's certainly feasible. I don't know whether it'd be preferable. Currently in
this CL I tried to minimize the potential impact to the existing trybot
configurations, code, etc. by making the additional_mirrors "mixin" optional.
Happy to change it (or even change all of these trybots' configurations into
lists of dictionaries, and change the required format).

[Sergey Berezin, 2016/01/06 19:12:35]

To be clear, what I'm proposing is backward-compatible to other configs (no
changes necessary), and it'll likely simplify your code.

Specifically, if you add logic to interpret 'builder': { ... } as 'builder': [{
... }] early on, you only need the code you already have for
'additional_mirrors', and can kill the special handling of the original fields.

But I'll leave it up to you and Pawel for a final form. My only concern is the
already non-trivial configuration grows yet another special flag, and it doesn't
really have to.

+          {
+            'mastername': 'chromium.gpu',
+            'buildername': 'GPU Linux Builder',
+            'tester': 'Linux Release (NVIDIA)',
+          },
+        ],
       },
       'linux_chromium_asan_rel_ng': {
         'mastername': 'chromium.memory',