Add helper to create random AsFPArgs for Ganesh unit tests

Reduces copy-paste and eases maintenance. I'll be adding another field to
AsFPArgs soon, and this is going to streamline that change.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3639

Change-Id: I6372ed5dce50a5ba9d73039bd4714e34502a1f75
Reviewed-on: https://skia-review.googlesource.com/3639
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/effects/gradients/SkRadialGradient.cpp b/src/effects/gradients/SkRadialGradient.cpp
index 5cf5245..8ad5880 100644
--- a/src/effects/gradients/SkRadialGradient.cpp
+++ b/src/effects/gradients/SkRadialGradient.cpp
@@ -314,11 +314,8 @@
                                      params.fStops, params.fColorCount, params.fTileMode) :
         SkGradientShader::MakeRadial(center, radius, params.fColors,
                                      params.fStops, params.fColorCount, params.fTileMode);
-    SkMatrix viewMatrix = GrTest::TestMatrix(d->fRandom);
-    auto dstColorSpace = GrTest::TestColorSpace(d->fRandom);
-    sk_sp<GrFragmentProcessor> fp = shader->asFragmentProcessor(SkShader::AsFPArgs(
-        d->fContext, &viewMatrix, NULL, kNone_SkFilterQuality, dstColorSpace.get(),
-        SkSourceGammaTreatment::kRespect));
+    GrTest::TestAsFPArgs asFPArgs(d);
+    sk_sp<GrFragmentProcessor> fp = shader->asFragmentProcessor(asFPArgs.args());
     GrAlwaysAssert(fp);
     return fp;
 }