Sprinkle some release asserts over HTMLFrameElementBase

      It appears to be difficult to get this right, but it's quite bad if we
get it wrong, so rather crash.

BUG=628942
R=dcheng@chromium.org

Review-Url: https://codereview.chromium.org/2169453002
Cr-Commit-Position: refs/heads/master@{#407430}