[freezing] Document Web Lock and IndexedDB opt-out.

Documenting the opt-out in preparation for sending the Intent to Ship
which will link to this page.

Bug: 980533
Change-Id: I3979d5a910ba75447b8b6dca0cf1ed0c63877c63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1837086
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702555}

chrome/browser/performance_manager/docs/freezing_opt_out_opt_in.md

@@ -49,6 +49,7 @@ On desktop, a page will not be frozen if:
 * The page is currently visible.
 * The page is currently being inspected by DevTools.
 * The page is currently capturing a window or screen.
+* The page is currently holding a Web Lock or an IndexedDB transaction.
 * The page is sharing its BrowsingInstance with another page.
 
 Pages which meet one of the above criteria are the ones that are the most likely