Revert "Remove SkLights include from SkCanvas.h"

Revert "Remove SkLights include from SkCanvas.h"

This reverts commit 9ff301bf918cd85d9f67047f10631b7a74345a90.

Reason for revert: need to update G3, Flutter.

Original change's description:
> Remove SkLights include from SkCanvas.h
>
> SkLights.h pulls in a bunch of other headers and is not needed (fwdecl
> works fine).
>
> Change-Id: Id2d7176eb3bf4609f72f46d513eebf59318f542f
> Reviewed-on: https://skia-review.googlesource.com/14904
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Florin Malita <fmalita@chromium.org>
>

TBR=mtklein@google.com,fmalita@chromium.org,reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I4799ad5b31aaeaf529c8b912bbe09aa8869a5e6c
Reviewed-on: https://skia-review.googlesource.com/15107
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>