Replace Selection.collapse() with Selection.removeAllRanges() in layout tests.

There are some tests calling Selection.collapse(null).
Selection.collapse(null) clears the current selection and its behavior is essentially same as Selection.removeAllRanges().
However, the Selection.collapse method is intended to create a range forming a caret:
https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#dom-selection-collapse
Calling Selection.collapse(null) to clear the selection is non-standard and I'm going to change Selection.idl to restrict such a usage.

BUG=361941

Review URL: https://codereview.chromium.org/236693002

git-svn-id: svn://svn.chromium.org/blink/trunk@171436 bbb929c8-8fbe-4397-9dbb-9b2b20218538