Enable preload=metadata lazy load on CrOS, leave off for macOS.

BUG=879406
TBR=liberato

Change-Id: I9a25b69c03edb29e35fcdcb9eb5f69cf7626b461
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627663Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662806}

media/base/media_switches.cc

@@ -199,14 +199,11 @@ const base::Feature kPictureInPicture {
 #endif
 };
 
-// Only decode preload=metadata elements upon visibility. Experiment results
-// vary across platforms and are still being analyzed on macOS and CrOS.
-//
-// Android, Windows, and Linux all saw improvements without regressions, so
-// enable by default there.
+// Only decode preload=metadata elements upon visibility. macOS still has some
+// undiagnosed performance and test issues, so isn't enabled yet.
 const base::Feature kPreloadMetadataLazyLoad {
   "PreloadMetadataLazyLoad",
-#if defined(OS_MACOSX) || defined(OS_CHROMEOS)
+#if defined(OS_MACOSX)
       base::FEATURE_DISABLED_BY_DEFAULT
 #else
       base::FEATURE_ENABLED_BY_DEFAULT