Add base/proc_utils.h for GetProcessId(), fix Windows build

This CL introduces a base::GetProcessId() to abstract getpid().
It also introduces a blessed api_test_support.h header to allow
the api_integrationtest.cc to indirectly use internal functions.

Bug: 132678367
Change-Id: I97743f103fedac555329a7ada7f4fdc6455dc203
diff --git a/Android.bp b/Android.bp
index 2779f9f..6961fed 100644
--- a/Android.bp
+++ b/Android.bp
@@ -891,6 +891,7 @@
     "src/tracing/internal/tracing_muxer_impl.cc",
     "src/tracing/platform.cc",
     "src/tracing/platform_posix.cc",
+    "src/tracing/test/api_test_support.cc",
     "src/tracing/trace_writer_base.cc",
     "src/tracing/tracing.cc",
     "src/tracing/track_event.cc",