GitLab

This CL is part of the effort to make Views default
constructible, allowing UI to be built in a declarative fashion.

* Default button listener to nullptr
* Add button listener setter
* Add unittest for setter

Bug: 1108460
Change-Id: I694acd0422b78951b1a7d17a90dcee3f8c8195d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327150Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792960}