Embedding-CSP: Refactoring directive strings into enum.

This is an effort as part of the experimental feature. Causing
patch: https://codereview.chromium.org/2474903002

BUG=647588

Review-Url: https://codereview.chromium.org/2516383002
Cr-Commit-Position: refs/heads/master@{#434474}
6 files changed