Simplify detection of new, test-related RenderFrameHosts.

Listening for WebContentsObserver::RenderFrameCreated is sufficient for
detecting new test frames - there is no need to also listen for
WebContentsObserver::RenderFrameHostChanged.

BUG=583485

Review-Url: https://codereview.chromium.org/2085023003
Cr-Commit-Position: refs/heads/master@{#401143}
6 files changed