Skip batch iteration in GrDrawTarget::forwardCombine when the max look ahead is 0

TBR=robertphillips@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2288373002

Review-Url: https://codereview.chromium.org/2288373002
diff --git a/src/gpu/GrDrawTarget.cpp b/src/gpu/GrDrawTarget.cpp
index 1889b87..dc2fd30 100644
--- a/src/gpu/GrDrawTarget.cpp
+++ b/src/gpu/GrDrawTarget.cpp
@@ -571,6 +571,9 @@
 }
 
 void GrDrawTarget::forwardCombine() {
+    if (fMaxBatchLookahead <= 0) {
+        return;
+    }
     for (int i = 0; i < fRecordedBatches.count() - 2; ++i) {
         GrBatch* batch = fRecordedBatches[i].fBatch.get();
         const SkRect& batchBounds = fRecordedBatches[i].fClippedBounds;