Move clip mask generation into GrReducedClip

This is a temporary solution to facilitate window rectangles and make
clip mask generation more accessible for testing. The eventual goal is
to simplify clips and merge GrReducedClip into GrClipStackClip.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2263343002

Review-Url: https://codereview.chromium.org/2263343002
diff --git a/src/gpu/GrContextPriv.h b/src/gpu/GrContextPriv.h
index 8646e25..29eb151 100644
--- a/src/gpu/GrContextPriv.h
+++ b/src/gpu/GrContextPriv.h
@@ -15,6 +15,8 @@
     data members or virtual methods. */
 class GrContextPriv {
 public:
+    GrDrawingManager* drawingManager() { return fContext->fDrawingManager; }
+
     // Create a drawContext that wraps an existing renderTarget
     sk_sp<GrDrawContext> makeWrappedDrawContext(sk_sp<GrRenderTarget> rt,
                                                 sk_sp<SkColorSpace> colorSpace,