Trace Processor: add --httpd

This CL adds a built-in HTTP RPC interface to
TraceProcessor. It allows to use a native trace
proocessor with the UI over HTTP.
It also move the RPC-related classes in a rpc/ subdir.

Bug: 143074239
Change-Id: I05416f8f604bafa4a9d26dbe0ff0db25fdd4d555
diff --git a/gn/perfetto.gni b/gn/perfetto.gni
index 1185ed4..a2ab7ef 100644
--- a/gn/perfetto.gni
+++ b/gn/perfetto.gni
@@ -233,6 +233,11 @@
   # Enables Fuchsia trace format support in trace processor.
   enable_perfetto_trace_processor_fuchsia =
       enable_perfetto_trace_processor && !build_with_chromium
+
+  # Enables httpd RPC support in the trace processor.
+  # Further per-OS conditionals are applied in gn/BUILD.gn.
+  enable_perfetto_trace_processor_httpd =
+      enable_perfetto_trace_processor && perfetto_build_standalone
 }
 
 # +---------------------------------------------------------------------------+