Moving gRPC support interfaces out of cronet and into a new component.

components/grpc_support/BUILD.gn

+source_set("grpc_support") {
+  sources = [
+    "bidirectional_stream.cc",
+    "bidirectional_stream.h",
+    "bidirectional_stream_c.cc",
+    "include/bidirectional_stream_c.h",
+  ]
+
+  deps = [
+    "//base",
+    "//net",
+    "//url",
+  ]
+}
+
+source_set("bidirectional_stream_unittest") {
+  testonly = true
+  sources = [
+    "bidirectional_stream_unittest.cc",
+  ]
+
+  deps = [
+    ":grpc_support",
+    "//base",
+    "//components/grpc_support/test:quic_test_server",
+    "//net",
+    "//net:test_support",
+  ]
+
+  public_deps = [
+    "//components/grpc_support/test:get_stream_engine_header",
+  ]
+}