Change string constant to constexpr char[] to unblock roll.

Should fix the "Chromium Binary Size" failures on for example
https://chromium-review.googlesource.com/c/chromium/src/+/5234153.

For the "Mutable Constants Added & Removed" check.

NOTRY=True

Bug: webrtc:15164
Change-Id: I5713e224018460edee5d4fd2d028c27834f46b00
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/336200
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41606}
diff --git a/pc/rtc_stats_collector.cc b/pc/rtc_stats_collector.cc
index a5a3067..6de174b 100644
--- a/pc/rtc_stats_collector.cc
+++ b/pc/rtc_stats_collector.cc
@@ -72,7 +72,7 @@
 const char kDirectionInbound = 'I';
 const char kDirectionOutbound = 'O';
 
-const char* kAudioPlayoutSingletonId = "AP";
+static constexpr char kAudioPlayoutSingletonId[] = "AP";
 
 // TODO(https://crbug.com/webrtc/10656): Consider making IDs less predictable.
 std::string RTCCertificateIDFromFingerprint(const std::string& fingerprint) {