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This CL fixes an issue with the page history. If an extension is removed
and the user tries to navigate back to the details page for that
extension, they may get caught in a loop (unable to go back). This CL
will navigate back or replace the history element (there are two different
cases: removing an extension from the details page; or navigating to an
invalid extension ID).

FYI, the case where the extension is removed from the main list page is
not affected (it doesn't have the issue).

Bug: 789891
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I45eb0f2917452da55e0e07c67356e24ae5bd56bb
Reviewed-on: https://chromium-review.googlesource.com/801975
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521483}