GitLab

Allows Checkbox class to use builder pattern, and converts the Checkbox
example to use the builder.

Also add callback property into Button class so Button class and its
subclasses can use callback in their metadata and builders.

Bug: 1130078
Change-Id: Ifd2e7cebd172d8fc91cbce50c0dacafd6b5ecf9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490339
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820069}