swarmbucket: fix empty dimension value

appengine/cr-buildbucket/proto/project_config.proto

Index: appengine/cr-buildbucket/proto/project_config.proto
diff --git a/appengine/cr-buildbucket/proto/project_config.proto b/appengine/cr-buildbucket/proto/project_config.proto
index 597954b3ddbb584be3489a1a85342eb94dce21c4..f63a0fd48daff6ec1e85fae745f25f251d105119 100644
--- a/appengine/cr-buildbucket/proto/project_config.proto
+++ b/appengine/cr-buildbucket/proto/project_config.proto
@@ -61,6 +61,9 @@ message Swarming {
     // "swarming_tag:builder:release"
     repeated string swarming_tags = 2;
     // Colon-delimited key-value pair of task dimensions.
+    //
+    // If value is not specified ("<key>:"), then it excludes a value for the
+    // same key specified in common_dimensions.
     repeated string dimensions = 3;
     // Specifies that a recipe to run.
     optional Recipe recipe = 4;
@@ -82,6 +85,9 @@ message Swarming {
   repeated string common_swarming_tags = 3;
 
   // Colon-delimited key-value pair of common task dimensions.
+  //
+  // If value is not specified ("<key>:"), then it excludes a value for the
+  // same key specified in swarming task template.
   repeated string common_dimensions = 4;
 
   // Configuration for each builder.