[Android settings] Add a menu item to export passwords

This CL adds a menu item, behind an off-by-default feature, for
exporting passwords from Chrome's settings. The menu item
currently does not perform any action.

Design: https://docs.google.com/a/chromium.org/document/d/1miKr2x0PTNIKgt3RQeur51ICQ66uszlYAmFXJONbG_0/edit?usp=sharing

Mocks: go chrome-pwd-export-mocks-android (Google-internal only)

Screenshots: https://crbug.com/788701#c1

Bug: 788701
Change-Id: I7e8b515596db4be061d494019a34220b9c019caf
Reviewed-on: https://chromium-review.googlesource.com/776681
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519607}
5 files changed