Reland ChromeOS: Enable ShelfModel synchronization by default.

This CL fixes flaky tests not caught by trybots on the first CL:
https://chromium-review.googlesource.com/c/chromium/src/+/682340

interactive_ui_tests: AppListTest.PressAppListButtonToShowAndDismiss
https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/45183

browser_tests: ShelfAppBrowserTest.ShelfButtonContextMenu
https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/45183

Original description:
Synchronize two ShelfModels between Ash and Chrome by default.
Can be disabled for debugging/testing in classic ash:
  --ash-disable-shelf-model-synchronization
  "Disable shelf model synchronization" (in about:flags)

Bug: 557406
Test: No Chrome OS shelf behavior changes; please file any bugs!
Change-Id: Iffeddde7425d246067434f1dd5bf3017bd23c1dd
Reviewed-on: https://chromium-review.googlesource.com/692749
Commit-Queue: Michael Wasserman <[email protected]>
Reviewed-by: James Cook <[email protected]>
Cr-Commit-Position: refs/heads/master@{#505532}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 815a976c..254dd5d 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1690,8 +1690,8 @@
 extern const char kAshShelfColorSchemeNormalMuted[];
 extern const char kAshShelfColorSchemeDarkMuted[];
 
-extern const char kAshEnableShelfModelSynchronization[];
-extern const char kAshEnableShelfModelSynchronizationDescription[];
+extern const char kAshDisableShelfModelSynchronization[];
+extern const char kAshDisableShelfModelSynchronizationDescription[];
 
 extern const char kMaterialDesignInkDropAnimationSpeedName[];
 extern const char kMaterialDesignInkDropAnimationSpeedDescription[];