Style cleanup: Spell out "element" rather than "elem" in WebSelectElement.cpp

BUG=none
TEST=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/25964002

git-svn-id: svn://svn.chromium.org/blink/trunk@158895 bbb929c8-8fbe-4397-9dbb-9b2b20218538