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Preload appears to trigger an edge-case crash in WebKit when a restore
is triggered and cancelled before it can complete.  This isn't specific
to preload, but is very easy to trigger in preload.  As a speculative
fix, if a preload is in restore, don't destroy it until after restore
is complete. This logic should really belong in WebState itself, so any
attempt to destroy a WebState during restore will trigger this logic.

Even better, this edge case crash should be fixed in WebKit:
    https://bugs.webkit.org/show_bug.cgi?id=217440.

The crash in WebKit appears to be related to IPC throttling.  Session
restore can create a large number of IPC calls, which can then be
throttled.  It seems if the WKWebView is destroyed with this backlog of
IPC calls, sometimes WebKit crashes.

See crbug.com/1032928 for an explanation for how to trigger this crash.

Note the timer should only be called if for some reason session
restoration fails to complete -- thus preventing a WebState leak.

Bug: 1032928
Change-Id: I217918437d5cdc6f610f3331634791699dd678ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461571Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815667}