cc: Fix data race in cc::TaskState::IsFinished.

cc/raster/tile_task.cc

Index: cc/raster/tile_task.cc
diff --git a/cc/raster/tile_task.cc b/cc/raster/tile_task.cc
index 87163164cf14ea01d54fc1d509b9c214aa8c11bc..d731433c2c0f25d6ec70d5b16b48bb8563b70a31 100644
--- a/cc/raster/tile_task.cc
+++ b/cc/raster/tile_task.cc
@@ -9,13 +9,26 @@
 namespace cc {
 
 TileTask::TileTask(bool supports_concurrent_execution)
-    : supports_concurrent_execution_(supports_concurrent_execution) {}
+    : supports_concurrent_execution_(supports_concurrent_execution),
+      did_complete_(false) {}
 
 TileTask::TileTask(bool supports_concurrent_execution,
                    TileTask::Vector* dependencies)
     : supports_concurrent_execution_(supports_concurrent_execution),
-      dependencies_(std::move(*dependencies)) {}
+      dependencies_(std::move(*dependencies)),
+      did_complete_(false) {}
+
+TileTask::~TileTask() {
+  DCHECK(did_complete_);
+}
+
+void TileTask::DidComplete() {
+  DCHECK(!did_complete_);
+  did_complete_ = true;
+}
 
-TileTask::~TileTask() {}
+bool TileTask::HasCompleted() const {
+  return did_complete_;
+}
 
 }  // namespace cc