Drop LABEL element from form-associated elements.

- Remove 'form' content attribute support from LABEL element
- 'form' IDL attribute returns the value of label.control.form.

Intent to implement and ship:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/QWyCpJjgfY8/TOxG9n-OAQAJ

BUG=607868

Review-Url: https://codereview.chromium.org/2000423006
Cr-Commit-Position: refs/heads/master@{#396149}
23 files changed