Reland "Define WEBRTC_ENABLE_SYMBOL_EXPORT if is_component_build=true."

This is a reland of 03bc15c646d5b41d3169f2686316944788f640ed

I will revert this reland as soon as it lands because I just need
to have a WebRTC commit to pin in Chromium in order to test the
component build (this CL enables symbol exports).

Original change's description:
> 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}

TBR: kwiberg@webrtc.org
No-Try: True
Bug: webrtc:9419
Change-Id: I4365fd66968218979e16954c7b00e3bb6d4fe806
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159024
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29723}

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 73569c5..98ff6a0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -111,7 +111,7 @@
   cflags = []
   ldflags = []
 
-  if (rtc_enable_symbol_export) {
+  if (rtc_enable_symbol_export || is_component_build) {
     defines = [ "WEBRTC_ENABLE_SYMBOL_EXPORT" ]
   }