thread_pool: constexpr TaskSourceSortKey()

This CL is intended to be trivial. It makes `TaskSourceSortKey`
`constexpr`, bringing along a `std::array` (in a unit test) for the
ride.

Change-Id: Ie8d33c428b8f39b9bf483e0d18def42960f61d60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6290268
Reviewed-by: Sean Maher <spvw@chromium.org>
Auto-Submit: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423526}

base/task/thread_pool/task_source_sort_key.cc

@@ -9,12 +9,6 @@ namespace base::internal {
 static_assert(sizeof(TaskSourceSortKey) <= 2 * sizeof(uint64_t),
               "Members in TaskSourceSortKey should be ordered to be compact.");
 
-TaskSourceSortKey::TaskSourceSortKey(TaskPriority priority,
-                                     TimeTicks ready_time,
-                                     uint8_t worker_count)
-    : priority_(priority),
-      worker_count_(worker_count),
-      ready_time_(ready_time) {}
 
 bool TaskSourceSortKey::operator<(const TaskSourceSortKey& other) const {
   // This TaskSourceSortKey is considered more important than |other| if it has

base/task/thread_pool/task_source_sort_key.h

@@ -15,10 +15,13 @@ namespace internal {
 // An immutable but assignable representation of the priority of a Sequence.
 class BASE_EXPORT TaskSourceSortKey final {
  public:
-  TaskSourceSortKey() = default;
-  TaskSourceSortKey(TaskPriority priority,
-                    TimeTicks ready_time,
-                    uint8_t worker_count = 0);
+  constexpr TaskSourceSortKey() = default;
+  constexpr TaskSourceSortKey(TaskPriority priority,
+                              TimeTicks ready_time,
+                              uint8_t worker_count = 0)
+      : priority_(priority),
+        worker_count_(worker_count),
+        ready_time_(ready_time) {}
 
   TaskPriority priority() const { return priority_; }
   uint8_t worker_count() const { return worker_count_; }

base/task/thread_pool/task_source_sort_key_unittest.cc

@@ -16,7 +16,7 @@ namespace base::internal {
 namespace {
 
 // Keys are manually ordered from the least important to the most important.
-const auto kTestKeys = std::to_array<TaskSourceSortKey>({
+constexpr auto kTestKeys = std::to_array<TaskSourceSortKey>({
     {TaskPriority::BEST_EFFORT, TimeTicks() + Seconds(2000)},
     {TaskPriority::BEST_EFFORT, TimeTicks() + Seconds(1000)},
     {TaskPriority::USER_VISIBLE, TimeTicks() + Seconds(2000), 1},