Make arguments to Element methods non-optional (except setAttribute)

This partially re-lands <https://codereview.chromium.org/105883012/>,
which was reverted in <https://codereview.chromium.org/120373003> because
making the second argument to setAttribute broke Twitter.

The second argument to setAttribute is left optional, making it
non-optional will be attempted again separately.

BUG=325922

Review URL: https://codereview.chromium.org/133683004

git-svn-id: svn://svn.chromium.org/blink/trunk@164971 bbb929c8-8fbe-4397-9dbb-9b2b20218538