unified ASTLayout/Layout and ASTModifiers/Modifiers

BUG=skia:

Change-Id: Ib4c2c94401e586e93e926776e13c0f7c12212f7e
Reviewed-on: https://skia-review.googlesource.com/5268
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
12 files changed