Remove excessive indentation in HTMLElement.idl

It's been like this for a long time for no apparent reason, and
is harder to read (and fit on screen) than it needs to be.

BUG=none

Review URL: https://codereview.chromium.org/26473003

git-svn-id: svn://svn.chromium.org/blink/trunk@159328 bbb929c8-8fbe-4397-9dbb-9b2b20218538