Turn on preservesPitch by default

This CL turns on the HTMLMediaElement.preservesPitch feature flag by
default.

Blink-dev discussion thread:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/XVCt0EXVplQ

WPT tests:
https://wpt.fyi/results/html/semantics/embedded-content/media-elements/preserves-pitch.html

Bug: 1096238
Change-Id: Ic22c5dbf7e48bafa630b43da210cbda3ba951c9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278584
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Daniel Bratell <bratell.d@gmail.com>
Reviewed-by: Daniel Bratell <bratell.d@gmail.com>
Cr-Commit-Position: refs/heads/master@{#792072}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1114,7 +1114,7 @@
     },
     {
       name: "MediaPreservesPitch",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "MediaQueryNavigationControls",

third_party/blink/web_tests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -396,6 +396,7 @@ html element audio
     property playbackRate
     property played
     property preload
+    property preservesPitch
     property readyState
     property remote
     property seekable
@@ -1136,6 +1137,7 @@ html element video
     property playsInline
     property poster
     property preload
+    property preservesPitch
     property readyState
     property remote
     property requestPictureInPicture

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3149,6 +3149,7 @@ interface HTMLMediaElement : HTMLElement
     getter playbackRate
     getter played
     getter preload
+    getter preservesPitch
     getter readyState
     getter remote
     getter seekable
@@ -3183,6 +3184,7 @@ interface HTMLMediaElement : HTMLElement
     setter onwaitingforkey
     setter playbackRate
     setter preload
+    setter preservesPitch
     setter src
     setter srcObject
     setter volume