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/BUILD.gn b/api/BUILD.gn
index d82c28d..45f555c 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -145,6 +145,7 @@
     "peer_connection_interface.cc",
     "peer_connection_interface.h",
     "peer_connection_proxy.h",
+    "proxy.cc",
     "proxy.h",
     "rtp_receiver_interface.cc",
     "rtp_receiver_interface.h",