[ios] Implemented LoadHtml w/o using data:// URLs.

This allows loading HTML and setting a specific URL (last part is needed
by suggestions tests, which currently use private API to set the URL).

BUG=619687

Review-Url: https://codereview.chromium.org/2295053003
Cr-Commit-Position: refs/heads/master@{#416151}
6 files changed