Update flag name of tab switcher on return

The word "enable" in the flag name is redundant.

Bug: 898685
Change-Id: I6c0af0dc0ae6a0ce01dc175ad193a60cd363767f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275345
Auto-Submit: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784179}

chrome/browser/flag_descriptions.cc

@@ -1181,7 +1181,7 @@ const char kHorizontalTabSwitcherAndroidDescription[] =
     "Changes the layout of the Android tab switcher so tabs scroll "
     "horizontally instead of vertically.";
 
-const char kTabSwitcherOnReturnName[] = "Enable tab switcher on return";
+const char kTabSwitcherOnReturnName[] = "Tab switcher on return";
 const char kTabSwitcherOnReturnDescription[] =
     "Enable tab switcher on return after specified time has elapsed";