Prefix ENABLE_RTC_EVENT_LOG with WEBRTC_.

Since this macro can be considered public, it makes sense to prefix it
with WEBRTC_ (also to avoid potential conflicts with client code).

This CL also removes some definitions of this macro in order to define
it only where it is strictly needed (it is only used in a .cc file).

Bug: webrtc:11142
Change-Id: Idce7389301e71d8434e238b3cf4ceaa9cf97cd87
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161008
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29957}

logging/BUILD.gn

@@ -197,7 +197,6 @@ rtc_library("rtc_event_log_impl_encoder") {
   ]
 
   if (rtc_enable_protobuf) {
-    defines += [ "ENABLE_RTC_EVENT_LOG" ]
     deps += [
       ":ice_log",
       ":rtc_event_audio",
@@ -340,7 +339,6 @@ if (rtc_enable_protobuf) {
     rtc_library("rtc_event_log_tests") {
       testonly = true
       assert(rtc_enable_protobuf)
-      defines = [ "ENABLE_RTC_EVENT_LOG" ]
       sources = [
         "rtc_event_log/encoder/blob_encoding_unittest.cc",
         "rtc_event_log/encoder/delta_encoding_unittest.cc",