[webnfc] Add WebNFC feature flag

This CL adds WebNFC feature flag, so that API access can be controlled
separately from other experimental web platform features. This would
enable feature control during origin-trial process.

BUG=520391

Review-Url: https://codereview.chromium.org/2830643002
Cr-Commit-Position: refs/heads/master@{#465992}
12 files changed