Bug 1305877 - Change WPT chunking default to directory hash; r=jgraham

Before this, the default of "equal_time" resulted in W7 jobs on Windows
debug builds executing over 6 times slower than the shortest chunk.
Chunking by hash of test directory makes the chunk sizes more consistent.
They aren't as consistent as a full path hashing or random chunking. But
it is an improvement.

MozReview-Commit-ID: 7bUSxY8GhH7

--HG--
extra : rebase_source : daa42b3e757a18fef1233338c0686154b387851d
extra : source : c4ae7809108445a184168a62dac6dab894c7f292

testing/web-platform/harness/wptrunner/wptcommandline.py

@@ -297,7 +297,7 @@ def check_args(kwargs):
 
     if kwargs["chunk_type"] is None:
         if kwargs["total_chunks"] > 1:
-            kwargs["chunk_type"] = "equal_time"
+            kwargs["chunk_type"] = "dir_hash"
         else:
             kwargs["chunk_type"] = "none"