Add new glic api boot injection code

See bug for motivation.

Updates the injected source to not use 'export', and to install
the boot function on window. This allows the client to add
the script as a new <script> element, and access the API.

This change does not break existing clients.

Bug: 382121207
Change-Id: I6cf7c5bb0edb4a4a66da321646ff60fd68ff76b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6088472
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1397555}