Prevent screenshots of the card editor views.

Before this patch, users could take screenshots of the local credit card
editor views, which show the full card number in plain text. (Chrome
does not show the full number of the server cards.)

This patch adds a FLAG_SECURE to the window of the local card editors
for PaymentRequest and autofill. The flag is added for Chrome Beta and
Stable builds to protect regular users. Dev, Canary, and developer
builds do not have the FLAG_SECURE set.

After this patch, users cannot take screenshots of the local card
editor views in Chrome Beta and Stable.

Bug: 721579
Change-Id: I82dcd7c83cec85fb3f8dd58aa7ab25c2826641d0
Reviewed-on: https://chromium-review.googlesource.com/504787
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#471807}