[Audio Process] Enable Kill-on-Hang by default on Linux.

There has been a recent increase in occasions where the audio process
has been hanging on Linux. To remedy the issue we are launching the
Kill-on-Hang feature by default on Linux. The feature has been running on
Windows and Mac for the past several months, and proved to be effective
in these situations.

Bug: 1043040
Change-Id: I2cb79a518faabc08a7c9b02890e9042abfd67c06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2081818Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Armando Miraglia <armax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745896}

media/audio/audio_features.cc

@@ -11,7 +11,7 @@ namespace features {
 // detected. It will be restarted when needed.
 const base::Feature kAudioServiceOutOfProcessKillAtHang{
   "AudioServiceOutOfProcessKillAtHang",
-#if defined(OS_WIN) || defined(OS_MACOSX)
+#if defined(OS_WIN) || defined(OS_MACOSX) || defined(OS_LINUX)
       base::FEATURE_ENABLED_BY_DEFAULT
 #else
       base::FEATURE_DISABLED_BY_DEFAULT