Add support for repeated keys in TaskRequest.

appengine/swarming/doc/Schemas.md

 # Schemas
 
 This page documents schemas used for tasks and bots in the DB.
 
 
 ## Tasks
 
 The task description core has to be read in order like a story in 4 parts; each
 block depends on the previous ones:
 
@@ skipping 182 matching lines @@
 
 AppEngine's automatic key numbering is never used. The entities are directly
 created with predefined keys so entity sharding can be tightly controlled to
 reduce DB contention.
 
   - TaskRequest has almost monotonically decreasing key ids based on time NOT'ed
     to make it decreasing. See task_request.py for more detail.
   - TaskResultSummary has key id = 1.
   - TaskRunResult has monotonically increasing key id starting at 1.
   - TaskToRun has the key id as `dimensions_hash` value is calculated as an
-    int32 from the TaskRequest.properties.dimensions dictionary.
+    int32 from the TaskRequest.properties.dimensions list of tuples (key,
+    value).
   - PerformanceStats has key id = 1.
   - BotTaskDimensions and TaskDimensions have key id `dimensions_hash`. This
     value is calculated as an int32 from the TaskRequest.properties.dimensions
-    dictionary.
+    list of tuples (key, value).
 
 
 ## Notes
 
   - Each root entity is tagged as Root.
   - Each line is annotated with the file that define the entities on this line.
   - Dotted line means a similar key relationship without actual entity
     hierarchy.