Have ContainerNodeAlgorithms deal with references instead of pointers

Have ContainerNodeAlgorithms deal with references instead of pointers
since those can never be null. This makes the code look safer.

R=adamk

Review URL: https://codereview.chromium.org/49153006

git-svn-id: svn://svn.chromium.org/blink/trunk@160769 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Update transitions/cubic-bezier-overflow-svg-length.html to use stroke-dashoffset

This test attempts to test that transitions with cubic-bezier timing functions
are able to produce values for SVG length properties that are outside the range
of the 'from' and 'to' keyframes. It uses the stroke-width property to do this,
but this property should not permit negative values. This is enforced by the
Web Animations implementation, but not the legacy implementation, which
complicates the test.

This change switches the test to use the stroke-dashoffset property, which
permits negative values.

Note that clamping of stroke-width values is tested in
animations/interpolation/svg-stroke-width-interpolation.html.

R=timloh

Review URL: https://codereview.chromium.org/41773003

git-svn-id: svn://svn.chromium.org/blink/trunk@160770 bbb929c8-8fbe-4397-9dbb-9b2b20218538