Work around signed-unsigned comparison warning.

Change-Id: I751e69ad74fd1ccf27c3533c83de409aea97df36
Reviewed-on: https://skia-review.googlesource.com/135265
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
diff --git a/src/gpu/ops/GrDrawVerticesOp.cpp b/src/gpu/ops/GrDrawVerticesOp.cpp
index 55d97f6..4103387 100644
--- a/src/gpu/ops/GrDrawVerticesOp.cpp
+++ b/src/gpu/ops/GrDrawVerticesOp.cpp
@@ -276,7 +276,7 @@
         return false;
     }
 
-    if (fVertexCount + that->fVertexCount > UINT16_MAX) {
+    if (fVertexCount + that->fVertexCount > SkTo<int>(UINT16_MAX)) {
         return false;
     }