Do not inherit TaskPriority in TaskTraits.

base/task_scheduler/task_traits.h

Index: base/task_scheduler/task_traits.h
diff --git a/base/task_scheduler/task_traits.h b/base/task_scheduler/task_traits.h
index 435fdac9af3843d18cec36bd441dacb6e74f91cf..61a1d548fd442015d9835afde00b0d2ef4eb8c8f 100644
--- a/base/task_scheduler/task_traits.h
+++ b/base/task_scheduler/task_traits.h
@@ -142,6 +142,9 @@ class BASE_EXPORT TaskTraits {
   // Returns true if tasks with these traits may use base/ sync primitives.
   bool with_base_sync_primitives() const { return with_base_sync_primitives_; }
 
+  // Returns true if the priority was set explicitly.
+  bool priority_set_explicitly() const { return priority_set_explicitly_; }
+
   // Returns the priority of tasks with these traits.
   TaskPriority priority() const { return priority_; }
 
@@ -149,8 +152,9 @@ class BASE_EXPORT TaskTraits {
   TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
 
  private:
-  bool may_block_;
-  bool with_base_sync_primitives_;
+  bool may_block_ : 1;
+  bool with_base_sync_primitives_ : 1;
+  bool priority_set_explicitly_ : 1;

[gab, 2017/04/25 17:16:41]

Let's ask Etienne to make sure these bit optimizations are worth it (and not
causing undesired churn).

[etienneb, 2017/04/25 17:45:59]

Using 
bool x : 1;
bool y : 1;

This will use less memory space for every allocated objects.
But every access will use more code as the compiler need to emit code to extract
the bit.

So, this is a tradeoff between code size and memory size.

[fdoray, 2017/04/25 17:57:48]

Since there will be very few accesses (ignoring constexpr constructors) and a
few instances in memory at all times, I'd say it's worth it.

[etienneb, 2017/04/25 18:05:47]

If this is not "largely" used, I'll stay with the simple version.
"bool" and not "bool : 1"

[fdoray, 2017/04/25 18:40:25]

Done.

   TaskPriority priority_;
   TaskShutdownBehavior shutdown_behavior_;
 };