Remove -webkit-auto, -webkit-center, -webkit-left, -webkit-right.

These are values for text-align that align blocks in addition to
inline content. You can get the same centering affect with margin:auto.
If we want this functionality, we should implement it with the
same alignment APIs that we use for flexbox.

Delete unused methods/arguments from RenderBlockFlow.

No change in behavior. Just deleting dead code.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/708843002