Reland of Moves mojo_js_integration_tests into blink.

third_party/WebKit/Source/core/mojo/tests/JsToCpp.mojom

+module js_to_cpp;
+
+// This struct encompasses all of the basic types, so that they
+// may be sent from C++ to JS and back for validation.
+struct EchoArgs {
+  int64 si64;
+  int32 si32;
+  int16 si16;
+  int8  si8;
+  uint64 ui64;
+  uint32 ui32;
+  uint16 ui16;
+  uint8  ui8;
+  float float_val;
+  float float_inf;
+  float float_nan;
+  double double_val;
+  double double_inf;
+  double double_nan;
+  string? name;
+  array<string>? string_array;
+  handle<message_pipe>? message_handle;
+  handle<data_pipe_consumer>? data_handle;
+};
+
+struct EchoArgsList {
+  EchoArgsList? next;
+  EchoArgs? item;
+};
+
+interface ForTesting {};
+
+// Note: For messages which control test flow, pick numbers that are unlikely
+// to be hit as a result of our deliberate corruption of response messages.
+interface CppSide {
+  // Sent for all tests to notify that the JS side is now ready.
+  StartTest@88888888();
+
+  // Indicates end for echo, bit-flip, and back-pointer tests.
+  TestFinished@99999999();
+
+  // Responses from specific tests.
+  PingResponse();
+  EchoResponse(EchoArgsList list);
+
+  // Having an associated interface pointer in the message makes sure the
+  // message header version 2 is tested.
+  BitFlipResponse(EchoArgsList arg, associated ForTesting? not_used);
+
+  BackPointerResponse(EchoArgsList arg);
+};
+
+interface JsSide {
+  SetCppSide(CppSide cpp);
+
+  Ping();
+  Echo(int32 numIterations, EchoArgs arg);
+  BitFlip(EchoArgs arg);
+  BackPointer(EchoArgs arg);
+};