Navigation API: Fire dispose events when session history entries are removed by the browser process

[chromium-wpt-export-bot]

A frame may have relevant entries removed from the joint session history based on actions outside of its control. The two main cases for this are:

• User-initiated history changes (e.g., erasing some or all of their browsing history)
• Pruning forward entries based on a navigation (i.e., when a navigation appends to the joint session history, but we are not currently at the end of the history, we remove all forward entries before appending). In this case, the navigating frame can detect and handle this case for itself, but other frames need the browser process to notify them of dispose entries.

This CL detects when a key is entirely removed from the joint session history, and notify the appropriate renderer so that the navigation API can remove it from the entries() array and firing a dispose event.

Bug: 1241598 Change-Id: If4d5ca1edc66d42b0f2ede2b509058ab9cacc745 Reviewed-on: https://chromium-review.googlesource.com/3818183 WPT-Export-Revision: 7ba8018b39750fbf941ae1f266eec451f07090a7