Export symbols needed by the Chromium component build (part 8).

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: Ib2c29054b2ae008f5291bd3b762a504b18534326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/130513
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27410}

api/rtc_error.h

@@ -20,6 +20,7 @@
 #include "absl/strings/string_view.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -80,7 +81,7 @@ enum class RTCErrorType {
 //
 // Doesn't contain anything beyond a type and message now, but will in the
 // future as more errors are implemented.
-class RTCError {
+class RTC_EXPORT RTCError {
  public:
   // Constructors.