Prevent 'javascript:' URL execution in sandboxed frame.

[1] notes that Chrome is violating step 1 of [2] by allowing
`<iframe sandbox=allow-scripts src="javascript:alert(1)">` to execute
JavaScript in an origin distinct from its parent (due to sandboxing).
This patch closes that gap with Firefox.

[1]: https://github.com/w3c/webappsec-secure-contexts/issues/26#issuecomment-214801969
[2]: https://html.spec.whatwg.org/multipage/browsers.html#javascript-protocol

BUG=629083
R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2154213003
Cr-Commit-Position: refs/heads/master@{#406255}
2 files changed