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This is a retry of https://chromium-review.googlesource.com/c/chromium/src/+/908371,
with fixing the DCHECK (NOTREACHED) error. We were not sure when the
NOTREACHED is reached before the fix, but now this case becomes
deterministic (see the discussion at crbug.com/816533)

Note that this might still cause Telemetry regressions reported at
crbug.com/753071 before, but this is expected.

A document is leaked because AutofillAgent hold a lastly used element
even after navigation happens, and the document of the element remains
until a new (input) element is focused. This CL fixes this issue by
resetting the element in AutofillAgent when navigation happens.

Bug: 734427, 755489, 816533
Change-Id: I7eb2f9fb879d826297247c4c66e366ec0b73067c
Reviewed-on: https://chromium-review.googlesource.com/939224Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543015}