Change computeLength to take const RenderStyle*

The various computeLength functions all take two RenderStyle pointers
(style and rootStyle).
Mark those as const as no implementation of computeLength modifies the
style.

R=leviw@chromium.org
BUG=263253

Review URL: https://chromiumcodereview.appspot.com/19468005

git-svn-id: svn://svn.chromium.org/blink/trunk@154712 bbb929c8-8fbe-4397-9dbb-9b2b20218538