Add CHECKs when a single GpuMemoryBuffer plane is assumed

cc/raster/one_copy_tile_task_worker_pool.cc

Index: cc/raster/one_copy_tile_task_worker_pool.cc
diff --git a/cc/raster/one_copy_tile_task_worker_pool.cc b/cc/raster/one_copy_tile_task_worker_pool.cc
index 64da37e91d09e02e7596def3bab763b50ebc5ace..2d6f644a19d5e81ee19a197f00328dad096bc131 100644
--- a/cc/raster/one_copy_tile_task_worker_pool.cc
+++ b/cc/raster/one_copy_tile_task_worker_pool.cc
@@ -17,6 +17,7 @@
 #include "cc/resources/resource_pool.h"
 #include "cc/resources/scoped_resource.h"
 #include "gpu/command_buffer/client/gles2_interface.h"
+#include "ui/gfx/buffer_format_util.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 
 namespace cc {
@@ -338,6 +339,8 @@ OneCopyTileTaskWorkerPool::PlaybackAndScheduleCopyOnWorkerThread(
   gfx::GpuMemoryBuffer* gpu_memory_buffer =
       raster_resource_write_lock->GetGpuMemoryBuffer();
   if (gpu_memory_buffer) {
+    DCHECK_EQ(
+        1u, gfx::NumberOfPlanesForBufferFormat(gpu_memory_buffer->GetFormat()));
     void* data = NULL;
     bool rv = gpu_memory_buffer->Map(&data);
     DCHECK(rv);