Hide enable-experimental-crostini-ui flag if no Crostini support

This hides the flag from showing up in chrome://flags if the platform
itself does not have VM support. VM support is indicated by the
'Crostini' feature flag being passed into Chrome.

Bug: None
Test: Verified experimental-ui flag shows/hides based on Crostini flag
Change-Id: Ibebc731628e7f05efbbcc586fde923b7c0d30388
Reviewed-on: https://chromium-review.googlesource.com/1037915Reviewed-by: James Hawkins <jhawkins@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#555216}

chrome/browser/about_flags.cc

@@ -3941,6 +3941,13 @@ bool SkipConditionalFeatureEntry(const FeatureEntry& entry) {
       channel == version_info::Channel::STABLE) {
     return true;
   }
+
+  // enable-experimental-crostini-ui is only available for boards that have
+  // VM support, which is controlled by the Crostini feature.
+  if (!strcmp("enable-experimental-crostini-ui", entry.internal_name) &&
+      !base::FeatureList::IsEnabled(features::kCrostini)) {
+    return true;
+  }
 #endif  // defined(OS_CHROMEOS)
 
   // data-reduction-proxy-lo-fi and enable-data-reduction-proxy-lite-page