Make it possible to remove an Awakable with a specific "context" from a Dispatcher.

mojo/edk/system/awakable_list.cc

Index: mojo/edk/system/awakable_list.cc
diff --git a/mojo/edk/system/awakable_list.cc b/mojo/edk/system/awakable_list.cc
index 510b88a3551fd3c02e0727d19d2aee758cd7a49b..e419d40383a2e08be8101e05287178643fb49ef6 100644
--- a/mojo/edk/system/awakable_list.cc
+++ b/mojo/edk/system/awakable_list.cc
@@ -70,5 +70,20 @@ void AwakableList::Remove(Awakable* awakable) {
   awakables_.erase(last, awakables_.end());
 }
 
+void AwakableList::RemoveWithContext(Awakable* awakable, uint64_t context) {
+  // We allow a thread to wait on the same handle multiple times simultaneously,
+  // so we need to scan the entire list and remove all occurrences of |waiter|.
+  auto last = awakables_.end();
+  for (AwakeInfoList::iterator it = awakables_.begin(); it != last;) {
+    if (it->awakable == awakable && it->context == context) {
+      --last;
+      std::swap(*it, *last);
+    } else {
+      ++it;
+    }
+  }
+  awakables_.erase(last, awakables_.end());
+}
+
 }  // namespace system
 }  // namespace mojo