switched reactor to absl::flat_hash_map (flutter/engine#56845)

We don't need consistent pointers anymore since we are avoiding deleting
objects to minimize time in the writer lock

related pr with benchmarks: https://github.com/flutter/engine/pull/56844

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engine/src/flutter/impeller/renderer/backend/gles/BUILD.gn

@@ -104,5 +104,6 @@ impeller_component("gles") {
     "../../:renderer",
     "../../../shader_archive",
     "//flutter/fml",
+    "//flutter/third_party/abseil-cpp/absl/container:flat_hash_map",
   ]
 }

engine/src/flutter/impeller/renderer/backend/gles/reactor_gles.h

@@ -9,6 +9,7 @@
 #include <memory>
 #include <vector>
 
+#include "flutter/third_party/abseil-cpp/absl/container/flat_hash_map.h"
 #include "fml/closure.h"
 #include "impeller/base/thread.h"
 #include "impeller/renderer/backend/gles/handle_gles.h"
@@ -276,12 +277,10 @@ class ReactorGLES {
   std::map<std::thread::id, std::vector<Operation>> ops_ IPLR_GUARDED_BY(
       ops_mutex_);
 
-  // Make sure the container is one where erasing items during iteration doesn't
-  // invalidate other iterators.
-  using LiveHandles = std::unordered_map<const HandleGLES,
-                                         LiveHandle,
-                                         HandleGLES::Hash,
-                                         HandleGLES::Equal>;
+  using LiveHandles = absl::flat_hash_map<const HandleGLES,
+                                          LiveHandle,
+                                          HandleGLES::Hash,
+                                          HandleGLES::Equal>;
   mutable RWMutex handles_mutex_;
   LiveHandles handles_ IPLR_GUARDED_BY(handles_mutex_);
   int32_t handles_to_collect_count_ IPLR_GUARDED_BY(handles_mutex_) = 0;