Introduce the WorkQueue

The WorkQueue allow other threads to submit tasks to be executed in
the thread that the WorkQueue lives on.

The WorkQueue will cancel all the pending tasks when it gets
destructed, to make sure the tasks won't get called with
invalid references.

src/mbgl/util/work_queue.cpp

@@ -0,0 +1,37 @@
+#include <mbgl/util/work_queue.hpp>
+
+#include <mbgl/util/run_loop.hpp>
+
+namespace mbgl {
+namespace util {
+
+WorkQueue::WorkQueue() : runLoop(RunLoop::Get()) {
+}
+
+WorkQueue::~WorkQueue() {
+    assert(runLoop == RunLoop::Get());
+
+    // Cancel all pending WorkRequests.
+    while (!queue.empty()) {
+        queue.pop();
+    }
+}
+
+void WorkQueue::push(std::function<void()>&& fn) {
+    std::lock_guard<std::mutex> lock(queueMutex);
+
+    auto workRequest = runLoop->invokeCancellable(std::bind(&WorkQueue::pop, this, std::move(fn)));
+    queue.push(std::move(workRequest));
+}
+
+void WorkQueue::pop(const std::function<void()>& fn) {
+    assert(runLoop == RunLoop::Get());
+
+    fn();
+
+    std::lock_guard<std::mutex> lock(queueMutex);
+    queue.pop();
+}
+
+}
+}