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Since crrev.com/c/1540371 it's been safe to trigger a clear
browsing data restore during a slimnav restore.  This is because we
DetachFromWebView during clear browsing data, and reset the
|is_restore_session_in_progress| flag.

DetachFromWebView already supports extracting restore urls from history,
so there shouldn't be any data loss.

Additionally, the delay implementation is wrong, since it's valid for a
webstate to be suspended in the tab grid  while in a slimnav restore state.
In fact, on a restart, most tabs are in that state.  So triggering clear
browsing data when that suspended tab is the 'active' tab, will hang forever.

Bug: 961726
TEST=
1.) Open a few tabs and create some navigation history for each (at least 10 navigation items)
2.) Quit the app
3.) Launch the app and quickly clear browsing data

Change-Id: Ic0a67523f5a0e2a63f31061735c12a2a27abb09f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807025Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697361}