Small improvements to MixedContentChecker.

Just a couple of improvements to MixedContentChecker:
- Remove uneeded check for data protocol: it's already included in the protocols
checked in SecurityOrigin::isSecure.
- Improved check for localhost by name to be more complete.

BUG=607878,624275

Review-Url: https://codereview.chromium.org/2151473002
Cr-Commit-Position: refs/heads/master@{#406000}
4 files changed