Add RTC_ prefix to (D)CHECKs and related macros.

We must remove dependency on Chromium, i.e. we can't use Chromium's base/logging.h. That means we need to define these macros in WebRTC also when doing Chromium builds. And this causes redefinition.

Alternative solutions:
* Check if we already have defined e.g. CHECK, and don't define them in that case. This makes us depend on include order in Chromium, which is not acceptable.
* Don't allow using the macros in WebRTC headers. Error prone since if someone adds it there by mistake it may compile fine, but later break if a header in added or order is changed in Chromium. That will be confusing and hard to enforce.
* Ensure that headers that are included by an embedder don't include our macros. This would require some heavy refactoring to be maintainable and enforcable.
* Changes in Chromium for this is obviously not an option.

BUG=chromium:468375
NOTRY=true

Review URL: https://codereview.webrtc.org/1335923002

Cr-Commit-Position: refs/heads/master@{#9964}

webrtc/modules/rtp_rtcp/source/rtcp_packet.cc

@@ -670,7 +670,7 @@ rtc::scoped_ptr<RawPacket> RtcpPacket::Build() const {
         : called_(false), packet_(packet) {}
     virtual ~PacketVerifier() {}
     void OnPacketReady(uint8_t* data, size_t length) override {
-      CHECK(!called_) << "Fragmentation not supported.";
+      RTC_CHECK(!called_) << "Fragmentation not supported.";
       called_ = true;
       packet_->SetLength(length);
     }