Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrClipStackClip.cpp

Index: src/gpu/GrClipStackClip.cpp
diff --git a/src/gpu/GrClipStackClip.cpp b/src/gpu/GrClipStackClip.cpp
index 003b4a5d439bb63782dd1370ea6bc7a0f94b4172..6103fd4532ce3b000adc88cabe8ccf6d62c8aac0 100644
--- a/src/gpu/GrClipStackClip.cpp
+++ b/src/gpu/GrClipStackClip.cpp
@@ -413,16 +413,10 @@ sk_sp<GrTexture> GrClipStackClip::CreateAlphaClipMask(GrContext* context,
         return sk_sp<GrTexture>(texture);
     }
 
-    // There's no texture in the cache. Let's try to allocate it then.
-    GrPixelConfig config = kRGBA_8888_GrPixelConfig;
-    if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
-        config = kAlpha_8_GrPixelConfig;
-    }
-
     sk_sp<GrDrawContext> dc(context->makeDrawContext(SkBackingFit::kApprox,
                                                      reducedClip.width(),
                                                      reducedClip.height(),
-                                                     config, nullptr));
+                                                     kAlpha_8_GrPixelConfig, nullptr));
     if (!dc) {
         return nullptr;
     }