WebUI: Support "notify: true" in CrLitElement reactive properties.

This makes it easier to migrate PolymerElement elements that used
2-way bindings to Lit, by automatically firing '<property-name>-changed'
events when such properties change, which is how Polymer 2-way bindings
actually work under the cover.

Firing such events makes Lit children compatible with Polymer parents
that still use 2-way bindings.

(inspired by code snippet provided by rictic@)

Bug: 1503223
Change-Id: I2d0b5fabd668f67bb887025c5c86fd4be647db6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5256117
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1255379}

• chrome/test/data/webui/cr_elements/cr_lit_element_test.ts[diff]
• third_party/lit/v3_0/cr_lit_element.ts[diff]
• ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.ts[diff]