RTC_EXPORT missing symbols for Chromium's component build.

This CL adds a dependecy on rtc_base/system:rtc_export to rtc_event but
only when built as part of Chromium (since rtc::Event should not be
used outside of WebRTC).

It also adds other missing RTC_EXPORTS.

Bug: webrtc:9419
Change-Id: Ib338004a5404a6b3c7929e146c29ad42572632cc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159692
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29781}
diff --git a/modules/audio_processing/include/audio_processing.h b/modules/audio_processing/include/audio_processing.h
index 80474f8..f1242a7 100644
--- a/modules/audio_processing/include/audio_processing.h
+++ b/modules/audio_processing/include/audio_processing.h
@@ -246,9 +246,9 @@
   // top-level processing effects. Use during processing may cause undesired
   // submodule resets, affecting the audio quality. Use the RuntimeSetting
   // construct for runtime configuration.
-  struct Config {
+  struct RTC_EXPORT Config {
     // Sets the properties of the audio processing pipeline.
-    struct Pipeline {
+    struct RTC_EXPORT Pipeline {
       Pipeline();
 
       // Maximum allowed processing rate used internally. May only be set to