Replace Selection.collapse() with Selection.removeAllRanges() in other 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.

This CL is similar to previous one: https://src.chromium.org/viewvc/blink?view=rev&revision=171436.

BUG=361954

Review URL: https://codereview.chromium.org/235933015

git-svn-id: svn://svn.chromium.org/blink/trunk@171534 bbb929c8-8fbe-4397-9dbb-9b2b20218538