Replace Selection.setPosition with Selection.collapse in layout tests.

Selection.setPosition is an alias of Selection.collapse and is not a standard function.

We take a UMA of setPosition and I'm going to obsolete it.

BUG=

Review URL: https://codereview.chromium.org/268753006

git-svn-id: svn://svn.chromium.org/blink/trunk@173482 bbb929c8-8fbe-4397-9dbb-9b2b20218538