Add end to end tests for WebSockets in net/websockets/

Some things (notably proxies) are impossible to test using layout tests,
and expensive and difficult to test using browser_tests.

Add a way to do simple end-to-end tests in net_unittests.

BUG=441709
TEST=net_unittests

Review URL: https://codereview.chromium.org/722343003

Cr-Commit-Position: refs/heads/master@{#313045}

net/BUILD.gn

@@ -1262,6 +1262,7 @@ if (!is_android && !is_win && !is_mac) {
         "websockets/websocket_deflate_stream_test.cc",
         "websockets/websocket_deflater_test.cc",
         "websockets/websocket_errors_test.cc",
+        "websockets/websocket_end_to_end_test.cc",
         "websockets/websocket_extension_parser_test.cc",
         "websockets/websocket_frame_parser_test.cc",
         "websockets/websocket_frame_test.cc",

net/net.gypi

@@ -1718,6 +1718,7 @@
       'websockets/websocket_deflate_predictor_impl_test.cc',
       'websockets/websocket_deflate_stream_test.cc',
       'websockets/websocket_deflater_test.cc',
+      'websockets/websocket_end_to_end_test.cc',
       'websockets/websocket_errors_test.cc',
       'websockets/websocket_extension_parser_test.cc',
       'websockets/websocket_frame_parser_test.cc',