add SkTaskGroup::done()

This lets you check if the work's done without blocking.
Seems handy.

Change-Id: Ie27c7b6fe0d01262b6a777abbc18b0de108641c0
Reviewed-on: https://skia-review.googlesource.com/38120
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/core/SkTaskGroup.cpp

@@ -29,12 +29,16 @@ void SkTaskGroup::batch(int N, std::function<void(int)> fn) {
     }
 }
 
+bool SkTaskGroup::done() const {
+    return fPending.load(std::memory_order_acquire) == 0;
+}
+
 void SkTaskGroup::wait() {
     // Actively help the executor do work until our task group is done.
     // This lets SkTaskGroups nest arbitrarily deep on a single SkExecutor:
     // no thread ever blocks waiting for others to do its work.
     // (We may end up doing work that's not part of our task group.  That's fine.)
-    while (fPending.load(std::memory_order_acquire) > 0) {
+    while (!this->done()) {
         fExecutor.borrow();
     }
 }

src/core/SkTaskGroup.h

@@ -25,8 +25,11 @@ public:
     // Add a batch of N tasks, all calling fn with different arguments.
     void batch(int N, std::function<void(int)> fn);
 
-    // Block until all Tasks previously add()ed to this SkTaskGroup have run.
-    // You may safely reuse this SkTaskGroup after wait() returns.
+    // Returns true if all Tasks previously add()ed to this SkTaskGroup have run.
+    // It is safe to reuse this SkTaskGroup once done().
+    bool done() const;
+
+    // Block until done().
     void wait();
 
     // A convenience for testing tools.