Update layer-pre-rendering for Chrome flag

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
===================================================================
--- src/gpu/SkGpuDevice.cpp	(revision 14674)
+++ src/gpu/SkGpuDevice.cpp	(working copy)
@@ -1942,6 +1942,11 @@
 void SkGpuDevice::EXPERIMENTAL_optimize(SkPicture* picture) {
     SkPicture::AccelData::Key key = GPUAccelData::ComputeAccelDataKey();
 
+    const SkPicture::AccelData* existing = picture->EXPERIMENTAL_getAccelData(key);
+    if (NULL != existing) {
+        return;
+    }
+
     SkAutoTUnref<GPUAccelData> data(SkNEW_ARGS(GPUAccelData, (key)));
 
     picture->EXPERIMENTAL_addAccelData(data);
@@ -1970,6 +1975,10 @@
 
     const GPUAccelData *gpuData = static_cast<const GPUAccelData*>(data);
 
+    if (0 == gpuData->numSaveLayers()) {
+        return false;
+    }
+
     SkAutoTArray<bool> pullForward(gpuData->numSaveLayers());
     for (int i = 0; i < gpuData->numSaveLayers(); ++i) {
         pullForward[i] = false;