WebView Prerender: Implement new communication channel between host app and...

WebView Prerender: Implement new communication channel between host app and prerendered pages for tests

This CL introduces a new communication channel between a host app (Java)
and prerendered pages to notify lifecycle events on the prerendered
pages in tests.

1) Before this CL

AwPrerenderTest.java depended on onLoadResource() to detect a timing
where prerendering is triggered and it's ready for activation. However,
this was not appropriate for following reasons:

- We shouldn't call onLoadResource() during prerendering to avoid a
  hosted app from being confused. See https://crbug.com/340041665.

- onLoadResource() is called when a prerender request is sent. This
  doesn't work for No-Vary-Search header tests that will be added by
  subsequent CLs, as the tests need to wait until WebView receives a
  response for prerendering request to see the header. onLoadResource()
  is too early for them.

2) After this CL

This CL adds a new communication channel to notify the primary page that
a prerendered page starts executing JavaScript for tests. The timing should work
for the No-Vary-Search header tests to be added by subsequent CLs.

The communication channel is implemented in a tricky way, as
communication among a host app (Java), the primary page, and prerendered
pages are restricted. First, when a prerendered page starts executing
JavaScript, the page writes a data in `window.localStorage`. This data
is visible to the primary page as well. The primary page detects writes
on the storage using `window.onstorage` event, and then forwards it to
the host app using postMessage on WebMessage.

In summary:
  Prerendered page
    --(window.localStorage)--> Primary page
    --(postMessage)--> Host app

3) Alternative ways to implement communication channel

- WebMessage between the host app and the prerendered page: Messages
  posted by prerendered pages are deferred until activation, so
  WebMessage is not available for notifying lifecycle events on
  prerendered pages.

- addJavascriptInterface(): In the current implementation, prerendered
  pages can access interfaces injected by addJavascriptInterface() and
  notify the host app of lifecycle events. However, we plan to disallow
  this communication path (calling an injected interface will result in
  prerender cancellation). See https://crbug.com/340005446 for details.

Bug: 339588213, 340005446, 340041665
Change-Id: Id56f4cb99b3e49dfcdb6404cad57e6c4fc9b70f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526702


Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Ken Okada <kenoss@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300905}