Fixes #2091. Fixes an issue where `performUpdate` was doing double duty as (1) make the element complete a pending update synchronously and (2) override to control update timing. Here `scheduleUpdate()` is added to satisfy (2), reserving (1) for `performUpdate()`. Since  by default `scheduleUpdate()` just returns the value of `performUpdate()` this should not break any existing code. However, the docs are updated to recommend that for scheduling `scheduleUpdate()` be implemented in favor of `performUpdate()`.

Co-authored-by: Justin Fagnani <justinfagnani@google.com>