add pan-x and pan-y support to CSS touch-action parsing.
Made TouchAction set of flags in renderstyleconstants.h as we need to support TouchAction: auto | none |[pan-x || pan-y].
Also in future may add more custom value similar to IE.

BUG=321643
TBR=rbyers@chromium.org,eseidel@chromium.org

Review URL: https://codereview.chromium.org/87973002

git-svn-id: svn://svn.chromium.org/blink/trunk@163948 bbb929c8-8fbe-4397-9dbb-9b2b20218538