Temporarily make //media/webrtc part of the WebRTC implementation.

This change only affects component builds.

Since //media/webrtc is a component, it will statically link WebRTC code
when `is_component_build = true`.

WebRTC is going to support Chromium component builds soon, and in order
to make it happen WebRTC symbols are being tagged with RTC_EXPORT.

This CL temporarily defines WEBRTC_LIBRARY_IMPL also when compiling
//media/webrtc in order to avoid to expand RTC_EXPORT to
__declspec(dllimport) since WebRTC code is still statically linked into
the //media/webrtc component.

This will be removed as soon as Chromium will depend on a unique
WebRTC component.

Bug: webrtc:9419
Change-Id: I731bf55414a64f80bee4dbe276a556a3a289629a
Reviewed-on: https://chromium-review.googlesource.com/c/1261398Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597522}

media/webrtc/BUILD.gn

@@ -24,6 +24,14 @@ component("webrtc") {
 
   defines = [ "IS_MEDIA_WEBRTC_IMPL" ]
 
+  # TODO(bugs.webrtc.org/9419): Remove this when Chromium will depend on a
+  # WebRTC component instead of rtc_static_libraries. In this phase we are
+  # marking WebRTC symbols as visible, but since this target is a component
+  # too, it will statically link WebRTC code so it doesn't need to dllimport
+  # symbols (this config tells the build system that //media/webrtc is part
+  # of the WebRTC implementation).
+  configs += [ "//third_party/webrtc:library_impl_config" ]
+
   deps = [
     "//base",
     "//third_party/webrtc/modules/audio_processing",