[webnfc] Allow access to WebNFC API only from main frame

This CL adds functionality that checks whether browsing context
is a main frame (top-level browsing context) as specified in
https://w3c.github.io/web-nfc/#security-policies

New layout test is added to verify new functionality.
- third_party/WebKit/LayoutTests/nfc/nfc-block-iframe.html

BUG=520391

Review-Url: https://codereview.chromium.org/2657023005
Cr-Commit-Position: refs/heads/master@{#447729}
4 files changed