Disable ClangTidy namespace comments for short blocks.

Namespace comments don't assist readability for blocks that can easily
fit on a single screen.

Change-Id: I93cbebe8e51400dead794c9eb41cb1eaa86bf756
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/312639
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>

.clang-tidy

@@ -19,5 +19,7 @@ Checks: >
     readability-redundant-smartptr-get,
     readability-static-accessed-through-instance
 CheckOptions:
+  - key:             llvm-namespace-comment.ShortNamespaceLines
+    value:           40
   - key:             llvm-namespace-comment.SpacesBeforeComments
     value:           2

gm/pathfill.cpp

@@ -21,7 +21,7 @@ struct PathDY {
     SkPath   path;
     SkScalar dy;
 };
-} // namespace
+}
 
 typedef PathDY (*MakePathProc)();