move program descriptor generation to flush

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/829e1b80b1020b17f2078020c990e079b70c077c

Review URL: https://codereview.chromium.org/777673003
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index d0bff30..c88e1c8 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -469,7 +469,7 @@
     // deltas from previous state at draw time. This function does the
     // backend-specific flush of the state.
     // returns false if current state is unsupported.
-    virtual bool flushGraphicsState(const GrOptDrawState&, DrawType) = 0;
+    virtual bool flushGraphicsState(const GrOptDrawState&) = 0;
 
     // clears target's entire stencil buffer to 0
     virtual void clearStencil(GrRenderTarget* target) = 0;