Change FilterOperationResolver & TransformBuilder to take const RenderStyle*

FilterOperationResolver and TransformBuilder take RenderStyle pointers
but do not modify the styles in any way. Mark the style and rootStyle
pointers as const.

R=leviw@chromium.org
BUG=263253

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/19593009

git-svn-id: svn://svn.chromium.org/blink/trunk@154765 bbb929c8-8fbe-4397-9dbb-9b2b20218538