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This CL adds:
- Request for password serialization
- Writing passwords to a temporary file
- Handling of errors that can occur during writing
- Presenting a UIActivityViewController with options to save the file

Bug:789122

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I2963b9b425b5e6de62f0461c2d66cc19b2d61f22
Reviewed-on: https://chromium-review.googlesource.com/842623Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533275}