Introduce a FlatMap and FlatSet into WTF

third_party/WebKit/Source/wtf/BUILD.gn

Index: third_party/WebKit/Source/wtf/BUILD.gn
diff --git a/third_party/WebKit/Source/wtf/BUILD.gn b/third_party/WebKit/Source/wtf/BUILD.gn
index 84c330e53bf3966354922aee5d84683e1ec2dee9..e22490c44bc820787dd60e30f970ff9e75e8326f 100644
--- a/third_party/WebKit/Source/wtf/BUILD.gn
+++ b/third_party/WebKit/Source/wtf/BUILD.gn
@@ -76,6 +76,9 @@ component("wtf") {
     "DynamicAnnotations.h",
     "FilePrintStream.cpp",
     "FilePrintStream.h",
+    "FlatMap.h",
+    "FlatSet.h",
+    "FlatTable.h",
     "Forward.h",
     "Functional.h",
     "GetPtr.h",
@@ -312,6 +315,8 @@ test("wtf_unittests") {
     "ASCIICTypeTest.cpp",
     "AssertionsTest.cpp",
     "DequeTest.cpp",
+    "FlatMapTest.cpp",
+    "FlatSetTest.cpp",
     "FunctionalTest.cpp",
     "HashMapTest.cpp",
     "HashSetTest.cpp",
@@ -322,6 +327,7 @@ test("wtf_unittests") {
     "PassRefPtrTest.cpp",
     "RefPtrTest.cpp",
     "SaturatedArithmeticTest.cpp",
+    "SetPerfTest.cpp",
     "StringExtrasTest.cpp",
     "StringHasherTest.cpp",
     "TreeNodeTest.cpp",