Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrBlurUtils.cpp

Index: src/gpu/GrBlurUtils.cpp
diff --git a/src/gpu/GrBlurUtils.cpp b/src/gpu/GrBlurUtils.cpp
index 2b7540da0abc6d40829aa3ee7890995022527d3e..412deece078cbc679316f32901ff51311768a8d7 100644
--- a/src/gpu/GrBlurUtils.cpp
+++ b/src/gpu/GrBlurUtils.cpp
@@ -104,17 +104,10 @@ static sk_sp<GrTexture> create_mask_GPU(GrContext* context,
         sampleCnt = 0;
     }
 
-    // We actually only need A8, but it often isn't supported as a
-    // render target so default to RGBA_8888
-    GrPixelConfig config = kRGBA_8888_GrPixelConfig;
-    if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, sampleCnt > 0)) {
-        config = kAlpha_8_GrPixelConfig;
-    }
-
     sk_sp<GrDrawContext> drawContext(context->makeDrawContext(SkBackingFit::kApprox,
                                                               maskRect.width(), 
                                                               maskRect.height(),
-                                                              config,
+                                                              kAlpha_8_GrPixelConfig,
                                                               nullptr,
                                                               sampleCnt));
     if (!drawContext) {