Make GrGpuCommandBuffer infer its render target from first draw

This is a temporary workaround to allow removal of GrBatch::renderTarget().

Change-Id: Ic14710a369802064cf6446e8191a98ea3595556d
Reviewed-on: https://skia-review.googlesource.com/5342
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index 674e8e4..9fab21d 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -210,11 +210,9 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 GrGpuCommandBuffer* GrVkGpu::createCommandBuffer(
-            GrRenderTarget* target,
             const GrGpuCommandBuffer::LoadAndStoreInfo& colorInfo,
             const GrGpuCommandBuffer::LoadAndStoreInfo& stencilInfo) {
-    GrVkRenderTarget* vkRT = static_cast<GrVkRenderTarget*>(target);
-    return new GrVkGpuCommandBuffer(this, vkRT, colorInfo, stencilInfo);
+    return new GrVkGpuCommandBuffer(this, colorInfo, stencilInfo);
 }
 
 void GrVkGpu::submitCommandBuffer(SyncQueue sync) {