Make swarming dimensions for GPU bots explicit

scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py

Index: scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py
diff --git a/scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py b/scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py
index a8955fefba66ba208603d697147fec73023284c5..db9265dba5e9a7da1819141a5d2fd7bff0c96ec8 100644
--- a/scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py
+++ b/scripts/slave/recipe_modules/chromium_tests/chromium_gpu.py
@@ -280,6 +280,9 @@ SPEC = {
         'platform': 'mac',
       },
       'enable_swarming': True,
+      'swarming_dimensions': {
+        'os': 'Mac-10.10',
+      },
     },
     'Mac Retina Release': {
       'chromium_config': 'chromium',
@@ -340,6 +343,9 @@ SPEC = {
         'platform': 'mac',
       },
       'enable_swarming': True,
+      'swarming_dimensions': {
+        'os': 'Mac-10.10',
+      },
     },
     'Mac 10.10 Retina Debug (AMD)': {
       'chromium_config': 'chromium',
@@ -360,6 +366,9 @@ SPEC = {
         'platform': 'mac',
       },
       'enable_swarming': True,
+      'swarming_dimensions': {
+        'os': 'Mac-10.10',
+      },
     },
   },
 }