Add toHTMLFieldSetElement and toHTMLLegendElement, and use them.

We'd like to avoid bare static_cast.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23847003

git-svn-id: svn://svn.chromium.org/blink/trunk@157095 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r157094

http://src.chromium.org/viewvc/blink?view=revision&revision=157094

BUG=281202
TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/23461017

git-svn-id: svn://svn.chromium.org/blink/trunk@157096 bbb929c8-8fbe-4397-9dbb-9b2b20218538