Provide a preference to delete data from cloud.

For now, it is a debug preference, but we may make this a proper
user setting in the future. Thus note that the messaging is not
final.

Pref looks like :
1. https://screenshot.googleplex.com/OLxW0myWD9.png
2. https://screenshot.googleplex.com/YeqWYXGstw.png

Bug: 18106936
Change-Id: I88bbc1a2ee0c23a48ff3972a91d57c218b10bb65