Reland #3: Ensure WebView notifies desktop automation on creation, destruction, and...

Reland #3: Ensure WebView notifies desktop automation on creation, destruction, and change

Original issue: https://codereview.chromium.org/880063002
Previous reland attempt: https://codereview.chromium.org/895623003/
previous reland attempt 2: https://codereview.chromium.org/890013006/
- able to reproduce the flakes locally by running multiple copies of the same test in parallel.
- tracked the issues down to the following points/solutions:
1. in automation_node.js, we now fire a loadComplete event if we detect a loaded root within enableFrame's callback. Previously, there could be a situation where a tree was already finished loading so it never actually fires that load complete.
2. in automation_node.js, we now store the callback that fires childrenChanged and the loadComplete above before requesting the browser initiate tree only accessibility which sends down the data.
3. in chromevox_next_e2e_test_base.js, we now check that the url contains 'test' to avoid processing loadCompletes from other rootWebAreas (e.g. about:blank).

TBR=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/907973004

Cr-Commit-Position: refs/heads/master@{#315785}