Improvements to uses of base::SmallMap

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 9d04554dcc57cf6d22f6eb4c4bfcac65ad194d65..a67cf21910b72accc87951f2092896aa531cd443 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -16,7 +16,7 @@
 
 #include "base/auto_reset.h"
 #include "base/bind.h"
-#include "base/containers/small_map.h"
+#include "base/containers/flat_map.h"
 #include "base/json/json_writer.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
@@ -1140,10 +1140,10 @@ void LayerTreeHostImpl::RemoveRenderPasses(FrameData* frame) {
   DCHECK_GE(frame->render_passes.size(), 1u);
 
   // A set of RenderPasses that we have seen.
-  std::set<int> pass_exists;
+  base::flat_set<int> pass_exists;
   // A set of RenderPassDrawQuads that we have seen (stored by the RenderPasses
   // they refer to).
-  base::SmallMap<std::unordered_map<int, int>> pass_references;
+  base::flat_map<int, int> pass_references;
 
   // Iterate RenderPasses in draw order, removing empty render passes (except
   // the root RenderPass).