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/api/task_queue/BUILD.gn b/api/task_queue/BUILD.gn
index 5c1a1dc..7d44977 100644
--- a/api/task_queue/BUILD.gn
+++ b/api/task_queue/BUILD.gn
@@ -22,6 +22,7 @@
   deps = [
     "../../rtc_base:checks",
     "../../rtc_base:macromagic",
+    "../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/base:config",
     "//third_party/abseil-cpp/absl/base:core_headers",
     "//third_party/abseil-cpp/absl/strings",
diff --git a/api/task_queue/task_queue_base.h b/api/task_queue/task_queue_base.h
index 7e42bba..90b1efd 100644
--- a/api/task_queue/task_queue_base.h
+++ b/api/task_queue/task_queue_base.h
@@ -13,6 +13,7 @@
 #include <memory>
 
 #include "api/task_queue/queued_task.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread_annotations.h"
 
 namespace webrtc {
@@ -21,7 +22,7 @@
 // in FIFO order and that tasks never overlap. Tasks may always execute on the
 // same worker thread and they may not. To DCHECK that tasks are executing on a
 // known task queue, use IsCurrent().
-class RTC_LOCKABLE TaskQueueBase {
+class RTC_LOCKABLE RTC_EXPORT TaskQueueBase {
  public:
   // Starts destruction of the task queue.
   // On return ensures no task are running and no new tasks are able to start