Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true.

In order to land the component build support in Chromium, it is
easier to turn on symbols export every time that is_component_build=true
instead of setting rtc_enable_symbol_export=is_component_build in
Chromium (since is_component_build is not available in .gn).

rtc_enable_symbol_export is still kept in the mix in order to turn
on symbol exports in any case a shared library will be added to the
WebRTC build.

Bug: webrtc:9419
Change-Id: I5a7195826dea13d9a6f10a1160c35f2864bfa6c2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157108
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29540}

BUILD.gn

@@ -106,7 +106,7 @@ config("common_inherited_config") {
   cflags = []
   ldflags = []
 
-  if (rtc_enable_symbol_export) {
+  if (rtc_enable_symbol_export || is_component_build) {
     defines = [ "WEBRTC_ENABLE_SYMBOL_EXPORT" ]
   }