Enable kAudioFlexibleLoopbackForSystemLoopback

Switch to use FLEXIBLE_LOOPBACK to provide system-wide loopback
as it is more reliable than POST_MIX_LOOPBACK for chromecast and
screen capture.

(cherry picked from commit f88aa5b)

Bug: b:304316369, b:304312713, b:306569817
Change-Id: Ia8e2e04a5fdd79e9bd89d2d039bf78837eb46f28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4982985
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Li-Yu Yu <aaronyu@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1219127}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5015529
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/6045@{#1302}
Cr-Branched-From: 905e8bd-refs/heads/main@{#1204232}

media/base/media_switches.cc

@@ -564,10 +564,9 @@ BASE_FEATURE(kCrOSSystemVoiceIsolationOption,
              "CrOSSystemVoiceIsolationOption",
              base::FEATURE_DISABLED_BY_DEFAULT);
 
-// TODO(b/306569817): Enable by default
 BASE_FEATURE(kAudioFlexibleLoopbackForSystemLoopback,
              "AudioFlexibleLoopbackForSystemLoopback",
-             base::FEATURE_DISABLED_BY_DEFAULT);
+             base::FEATURE_ENABLED_BY_DEFAULT);
 #endif
 
 // Make MSE garbage collection algorithm more aggressive when we are under