Export symbols needed by the Chromium component build (part 1).

This CL uses RTC_EXPORT (defined in rtc_base/system/rtc_export.h)
to mark WebRTC symbols as visible from a shared library, this doesn't
mean these symbols are part of the public API (please continue to refer
to [1] for info about what is considered public WebRTC API).

[1] - https://webrtc.googlesource.com/src/+/HEAD/native-api.md

Bug: webrtc:9419
Change-Id: I802abd32874d42d3aa5ecd3c8022e7cf5e043d99
Reviewed-on: https://webrtc-review.googlesource.com/c/103505
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24969}

rtc_base/task_queue.h

@@ -18,6 +18,7 @@
 #include "absl/memory/memory.h"
 #include "rtc_base/constructormagic.h"
 #include "rtc_base/scoped_ref_ptr.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread_annotations.h"
 
 namespace rtc {
@@ -150,7 +151,7 @@ static std::unique_ptr<QueuedTask> NewClosure(Closure&& closure,
 // TaskQueue itself has been deleted or it may happen synchronously while the
 // TaskQueue instance is being deleted.  This may vary from one OS to the next
 // so assumptions about lifetimes of pending tasks should not be made.
-class RTC_LOCKABLE TaskQueue {
+class RTC_LOCKABLE RTC_EXPORT TaskQueue {
  public:
   // TaskQueue priority levels. On some platforms these will map to thread
   // priorities, on others such as Mac and iOS, GCD queue priorities.