Unreverting Embedding-CSP: Refactoring directive strings into enum.

Removing ASSERT_DEATH.

Origin Review-Url: https://codereview.chromium.org/2516383002
Reverted: https://codereview.chromium.org/2528133002/

BUG=647588

Review-Url: https://codereview.chromium.org/2532913002
Cr-Commit-Position: refs/heads/master@{#434655}
6 files changed