Never bypass CSP in WebUI pages

BUG=668645
TEST=browser_test --gtest_filter=ExtensionCSPBypassTest.LoadWebAccessibleScript

Change-Id: I03a96a742b06ca6ec81fad95aae17cbdb11e1cce
Reviewed-on: https://chromium-review.googlesource.com/567499
Commit-Queue: Rob Wu <rob@robwu.nl>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528343}
6 files changed