Prefix flag macros with WEBRTC_.

Macros defined in rtc_base/flags.h are intended to be used to define
flags in WebRTC's binaries (e.g. tests).

They are currently not prefixed and this could cause problems with
downstream clients since these names are quite common.

This CL adds the 'WEBRTC_' prefix to them.

Generated with:

for x in DECLARE DEFINE; do
  for y in bool int float string FLAG; do
    git grep -l "\b$x\_$y\b" | \
    xargs sed -i "s/\b$x\_$y\b/WEBRTC_$x\_$y/g"
  done
done
git cl format

Bug: webrtc:9884
Change-Id: I7b524762b6a3e5aa5b2fc2395edd3e1a0fe72591
Reviewed-on: https://webrtc-review.googlesource.com/c/106682
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25270}

modules/audio_coding/neteq/tools/rtp_jitter.cc

@@ -23,7 +23,7 @@ namespace webrtc {
 namespace test {
 namespace {
 
-DEFINE_bool(help, false, "Print help message");
+WEBRTC_DEFINE_bool(help, false, "Print help message");
 
 constexpr size_t kRtpDumpHeaderLength = 8;