Use localToContainerPoint when checking mapToContainer result

The mapToContainer method is calculating the value in relation
to the container, but when we verify the result we calculate
the localToAbsolute value. This doesn't take the container
into account so we get the wrong values.

BUG=371640

Review URL: https://codereview.chromium.org/310483002

git-svn-id: svn://svn.chromium.org/blink/trunk@175163 bbb929c8-8fbe-4397-9dbb-9b2b20218538