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Currently, the permission prompt bubble's title is "$origin wants to:",
which causes a confusing screenreader announcement when the bubble
appears: "Chrome has new window. $origin wants to."

This change introduces a new accessible title, which includes some of
the requested permissions. Specifically, it is one of:

* "$origin wants to: $permission"
* "$origin wants to: $permission and $permission"
* "$origin wants to: $permission, $permission, and more"

To accomplish that, this change:
1) Adds new strings for those messages
2) Adds logic to PermissionPromptBubbleView to build the accessible
   title
3) Slightly refactors PermissionPromptBubbleView to make it possible to
   construct without a live Browser and BrowserWindow, which allows...
4) Adds unit tests for PermissionPromptBubbleView to cover the new
   behavior

Bug: 434574
Change-Id: I3b72f1a51489776c059e952da470034e2e8b131b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116926
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Adrienne Porter Felt <felt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755122}