Fix the binary size regression on Chromium Windows.

There is a dependency chain from Chromium windows main_dll to Opus
which should never exist. We used to rely on rtc_static_library
to break this chain. So this CL replaced some rtc_source_set
with rtc_static_library.

libvpx fix (https://chromium-review.googlesource.com/c/544107/) for
ios-simulator linking issue is landed and this CL can be sumbitted once the new
Chromium is rolled into WebRTC.

BUG=chromium:734631

Review-Url: https://codereview.webrtc.org/2947273002
Cr-Commit-Position: refs/heads/master@{#18709}

webrtc/media/BUILD.gn

@@ -50,7 +50,7 @@ rtc_source_set("rtc_h264_profile_id") {
   ]
 }
 
-rtc_source_set("rtc_media_base") {
+rtc_static_library("rtc_media_base") {
   # TODO(kjellander): Remove (bugs.webrtc.org/6828)
   # Enabling GN check triggers cyclic dependency error:
   #   //webrtc/media:rtc_media_base ->

webrtc/pc/BUILD.gn

@@ -185,7 +185,7 @@ rtc_static_library("peerconnection") {
 # need should use CreateModularCreatePeerConnectionFactory instead, using the
 # "peerconnection" build target and other targets specific to their
 # requrements. See comment in peerconnectionfactoryinterface.h.
-rtc_source_set("create_pc_factory") {
+rtc_static_library("create_pc_factory") {
   sources = [
     "createpeerconnectionfactory.cc",
   ]