Remove Selection.containsNode assertion that is not valid.

Align with Firefox and return false in the case where the
Range method leads to an exception. Previously, we returned
true.

The behavior of these selection APIs are not in any specs
and they are only implemented by Blink and Firefox.

R=haraken@chromium.org, nbarth@chromium.org
BUG=

Review URL: https://codereview.chromium.org/212873002

git-svn-id: svn://svn.chromium.org/blink/trunk@170291 bbb929c8-8fbe-4397-9dbb-9b2b20218538