`about:blank` is not mixed content.

The refactoring in [1] broke our handling of `about:blank` by moving from
`SecurityOrigin::isSecure(url)` to `SecurityOrigin::create(url)`. The
latter treats contextually-bound URLs like `about:blank` as unique security
origins, which I completely missed when writing tests.

This patch corrects the error, and adds a unit test that I should have
added a long time ago. :)

[1]: https://chromium.googlesource.com/chromium/src/+/130ee686fa00b617bfc001ceb3bb49782da2cb4e

BUG=624275
R=yoav@yoav.ws

Review-Url: https://codereview.chromium.org/2123463002
Cr-Commit-Position: refs/heads/master@{#403655}
2 files changed