Try finding exact scratch texture in cache before approx, take 2

This is a reland of https://skia-review.googlesource.com/c/skia/+/42940
which was reverted since its parent CL had to be reverted. The parent
has since relanded.

Bug: skia:
Change-Id: Ibf4bf7ce0332e4faea9a6e95a40d1c1f07fb0e75
Reviewed-on: https://skia-review.googlesource.com/51244
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>

src/gpu/GrResourceProvider.cpp

@@ -187,6 +187,10 @@ sk_sp<GrTexture> GrResourceProvider::createApproxTexture(const GrSurfaceDesc& de
         return nullptr;
     }
 
+    if (auto tex = this->refScratchTexture(desc, flags)) {
+        return tex;
+    }
+
     SkTCopyOnFirstWrite<GrSurfaceDesc> copyDesc(desc);
 
     // bin by pow2 with a reasonable min