Use source_sets in component builds and static_library in release builds.

Static libraries don't guarantee that an exported symbol gets linked
into a shared library (and in order to support Chromium's component
build mode, WebRTC needs to be linked as a shared library).

Source sets always pass all the object files to the linker.

On the flip side, source_sets link more object files in release builds
and to avoid this, this CL introduces a the GN template "rtc_library" that
expands to static_library during release builds and to source_set during
component builds.

See: https://gn.googlesource.com/gn/+/master/docs/reference.md#func_source_set

Bug: webrtc:9419
Change-Id: I4667e820c2b3fcec417becbd2034acc13e4f04fe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157168
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#29525}
diff --git a/modules/BUILD.gn b/modules/BUILD.gn
index 621e64d..ece91af 100644
--- a/modules/BUILD.gn
+++ b/modules/BUILD.gn
@@ -39,7 +39,7 @@
   ]
 }
 
-rtc_source_set("module_api") {
+rtc_library("module_api") {
   visibility = [ "*" ]
   sources = [
     "include/module.h",