Change prealloc number of child FPs be 1 rather than 2

TBR=joshualitt@google.com

Review URL: https://codereview.chromium.org/1458913003 .
diff --git a/include/gpu/GrFragmentProcessor.h b/include/gpu/GrFragmentProcessor.h
index df6860e..55b2f90 100644
--- a/include/gpu/GrFragmentProcessor.h
+++ b/include/gpu/GrFragmentProcessor.h
@@ -211,7 +211,7 @@
     SkSTArray<4, const GrCoordTransform*, true>     fCoordTransforms;
     int                                             fNumTexturesExclChildren;
     int                                             fNumTransformsExclChildren;
-    SkSTArray<2, const GrFragmentProcessor*, true>  fChildProcessors;
+    SkSTArray<1, const GrFragmentProcessor*, true>  fChildProcessors;
 
     typedef GrProcessor INHERITED;
 };