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",