[Lacros a11y] ChromeVox injects gdocs script on startup

Google Docs (drive, sheets, etc) has an accessibility mode that
is required to be on to make pages accessible to screen readers,
including ChromeVox. ChromeVox in Ash informs gdocs tabs that
it is enabled by injecting an event, and ChromeVox in Lacros
does this for new gdocs tabs that are opened after ChromeVox
Helper is installed. This change ensures that gdocs tabs in Lacros
which were open before ChromeVox was enabled also have the
event injected, so that pre-existing Lacros gdocs tabs have
a11y mode turned on and automatically work with ChromeVox.

In short, this replicates behavior in Ash gdocs tabs from ChromeVox in
Lacros gdocs tabs, specifically see
chrome/browser/resources/chromeos/accessibility/chromevox/background/injected_script_loader.js.

For more, see go/lacros-embedded-a11y-extensions.

Also adds some required typescript definitions for the parts
of the extension API used by ChromeVox helper.

Bug: b/286413518
AX-Relnotes: Already open Lacros gdocs tabs turn on a11y when CVox enabled
Test: Manual (we don't have browsertests on live gdocs)
Change-Id: Ie361287b66da65bcbc8c64e600e8268b98e0873d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4652866
Reviewed-by: Jacob Francis <francisjp@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1164390}

• chrome/browser/resources/accessibility/chromevox_helper/BUILD.gn[diff]
• chrome/browser/resources/accessibility/chromevox_helper/service_worker.ts[Added - diff]
• chrome/browser/resources/accessibility/chromevox_helper_manifest.json.jinja2[diff]
• tools/typescript/definitions/runtime.d.ts[diff]
• tools/typescript/definitions/scripting.d.ts[Added - diff]
• tools/typescript/definitions/storage.d.ts[Added - diff]
• tools/typescript/definitions/windows.d.ts[diff]