Add option to control maximum GrBatch lookback

Review URL: https://codereview.chromium.org/1498653002
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 1c30d54..2b20e17 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -89,6 +89,7 @@
     GrDrawTarget::Options dtOptions;
     dtOptions.fClipBatchToBounds = options.fClipBatchToBounds;
     dtOptions.fDrawBatchBounds = options.fDrawBatchBounds;
+    dtOptions.fMaxBatchLookback = options.fMaxBatchLookback;
     fDrawingManager.reset(new GrDrawingManager(this, dtOptions));
 
     // GrBatchFontCache will eventually replace GrFontCache