Add unit test for vertex attribute count.

BUG=skia:
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Review URL: https://codereview.chromium.org/1826863002
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index ce15b41..177fbab 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -497,6 +497,7 @@
                  const GrMesh* meshes,
                  int meshCount) {
     if (primProc.numAttribs() > this->caps()->maxVertexAttributes()) {
+        fStats.incNumFailedDraws();
         return false;
     }
     this->handleDirtyContext();