Revert "Replaces SynchronousMethodCall with rtc::Thread::Invoke."

This reverts commit b0e0728159f07269a875c5b53658603cf6733480.

Reason for revert:

Causes Chromium tests to timeout, preventing rolls into Chromium.

Original change's description:
> Replaces SynchronousMethodCall with rtc::Thread::Invoke.
> 
> Given that we already have Thread:.Invoke that can be used with lambda,
> SynchronousMethodCall doesn't add any value.
> 
> This simplification prepares for simulated time peer connection tests.
> 
> Bug: webrtc:11255
> Change-Id: I478a11f15e30e009dae4a3fee2120f6d7a03355f
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165683
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#30217}

TBR=steveanton@webrtc.org,srte@webrtc.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: webrtc:11255
Change-Id: I9d3aa218013129db7a09a77500a0547ce9ae341a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166047
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Commit-Queue: Guido Urdaneta <guidou@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30265}
diff --git a/api/proxy.cc b/api/proxy.cc
new file mode 100644
index 0000000..e668285
--- /dev/null
+++ b/api/proxy.cc
@@ -0,0 +1,37 @@
+/*
+ *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "api/proxy.h"
+
+namespace webrtc {
+namespace internal {
+
+SynchronousMethodCall::SynchronousMethodCall(rtc::MessageHandler* proxy)
+    : proxy_(proxy) {}
+
+SynchronousMethodCall::~SynchronousMethodCall() = default;
+
+void SynchronousMethodCall::Invoke(const rtc::Location& posted_from,
+                                   rtc::Thread* t) {
+  if (t->IsCurrent()) {
+    proxy_->OnMessage(nullptr);
+  } else {
+    t->Post(posted_from, this, 0);
+    e_.Wait(rtc::Event::kForever);
+  }
+}
+
+void SynchronousMethodCall::OnMessage(rtc::Message*) {
+  proxy_->OnMessage(nullptr);
+  e_.Set();
+}
+
+}  // namespace internal
+}  // namespace webrtc