Fix more -Wsign-compare warnings in heap, mips, base, etc.

src/cancelable-task.cc

Index: src/cancelable-task.cc
diff --git a/src/cancelable-task.cc b/src/cancelable-task.cc
index 9bbddca6b24a9f8483d7665d6512907c064157d7..ea351f8908ce3694382ce1c597c4f46d2f81fb7e 100644
--- a/src/cancelable-task.cc
+++ b/src/cancelable-task.cc
@@ -44,7 +44,7 @@ void CancelableTaskManager::RemoveFinishedTask(uint32_t id) {
   base::LockGuard<base::Mutex> guard(&mutex_);
   size_t removed = cancelable_tasks_.erase(id);
   USE(removed);
-  DCHECK_NE(0, removed);
+  DCHECK_NE(0u, removed);
   cancelable_tasks_barrier_.NotifyOne();
 }