Tab Search: Set the ChromeOS flag for Tab Search to enabled

This CL sets the kTabSearch flag to be enabled by default on
ChromeOS and leaves it disabled by default on the remaining
desktop platforms (Linux, Mac, Windows).

This is part of the required launch cleanup for the ChromeOS
Tab Search experiment.

Launch config CL:
https://critique-ng.corp.google.com/cl/347894919

Bug: 1116182
Change-Id: I4de20453851b095a62c099a02c8dcebfb26ca00c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596029
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Yuheng Huang <yuhengh@chromium.org>
Reviewed-by: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838959}

chrome/browser/ui/ui_features.cc

@@ -105,8 +105,16 @@ const base::Feature kTabHoverCardImages{"TabHoverCardImages",
 const base::Feature kTabOutlinesInLowContrastThemes{
     "TabOutlinesInLowContrastThemes", base::FEATURE_DISABLED_BY_DEFAULT};
 
-// Enables searching tabs across multiple windows.
+// Enables searching tabs across multiple windows. This feature launch is
+// staggered to release to ChromeOS first and other platforms later. Tab Search
+// is enabled by default on ChromeOS following its launch on the platform.
+// TODO(crbug.com/1137558): Remove this after launch to the remaining desktop
+// platforms.
+#if defined(OS_CHROMEOS)
+const base::Feature kTabSearch{"TabSearch", base::FEATURE_ENABLED_BY_DEFAULT};
+#else
 const base::Feature kTabSearch{"TabSearch", base::FEATURE_DISABLED_BY_DEFAULT};
+#endif  // defined(OS_CHROMEOS)
 
 // Enables the tab search submit feedback button.
 const base::Feature kTabSearchFeedback{"TabSearchFeedback",