Detect any callers that delete a pending entry while it is still in use.

Any crashes of this form indicate a bug somewhere in the call stack, since
the navigation logic is not re-entrant.

BUG=347742
TEST=Just a second line of defense, no intended behavior change.
R=nasko@chromium.org, samarth@chromium.org

Review URL: https://codereview.chromium.org/181433019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254252 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254252