[Android password settings] Display warning before export

This CL adds a warning to be displayed before the user can export
their passwords.

Mocks:
https://docs.google.com/presentation/d/1nIm5OmaOnb85ZAwMZPSVqHT0vkFbsRYZhOzbmznWU_c/edit#slide=id.g289b1efcd8_0_21

Note: the mock shows slightly different styling, but with the UX
designer we settled on using a standard dialog style already existing
in Chromium and checking separately whether that style needs fixing,
rather than introducing inconsistent styling among Chrome's dialogs.

Design: go chrome-pwd-export

Screenshot: https://crbug.com/788701#c7

Bug: 788701
Change-Id: I3a2dbdc24429c5662fdc2b178630a8e45ba603a1
Reviewed-on: https://chromium-review.googlesource.com/803955
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522062}
5 files changed