Bug 1536471 - Consolidate calls to nsISHEntry::GetChildAt in nsDocShell::AddToSessionHistory by adding nsSHEntry::ClearEntry method, r=peterv, r=nika for adding sync IPC messages

Currently, nsDocShell repeatedly calls nsISHEntry::GetChildAt and
nsISHEntry::RemoveChild  in nsDocShell::AddToSessionHistory,
which results in twice as many IPC calls as the number of children
a session history entry has. Additionally, there is one extra
IPC sync call to nsISHEntry::AbandonBFCacheEntry after the loop.

With the proposed solution, there will only be 1 sync IPC call.

Differential Revision: https://phabricator.services.mozilla.com/D24978

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extra : rebase_source : ab98af417bcb28fdfcb1a115473b505fa8a70aa1
extra : source : 0270d6b952ffa1704690c777c55fcaed30125b38
extra : histedit_source : 9a031cb921ad8efa1bd96f7342e08337f3c0ca8a