crostini: New installs get Buster by default.

Test: CQ and local build
Bug: chromium:930901
Change-Id: I1784683e1ee55ff956d512cc34616a51148ba7aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914597
Commit-Queue: David Munro <davidmunro@google.com>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715190}

chromeos/constants/chromeos_features.cc

@@ -53,7 +53,7 @@ const base::Feature kCrostiniBackup{"CrostiniBackup",
 
 // Enables or disables Crostini using Buster container images.
 const base::Feature kCrostiniUseBusterImage{"CrostiniUseBusterImage",
-                                            base::FEATURE_DISABLED_BY_DEFAULT};
+                                            base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables or disables Crostini GPU support.
 const base::Feature kCrostiniGpuSupport{"CrostiniGpuSupport",