[Instant Start] Update human-readable flag name

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

Bug: 1081618
Change-Id: Id95be4ac1ccb63c79c00785d67a21622a94a2698
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2275634
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@{#784178}

chrome/browser/flag_descriptions.cc

@@ -2625,7 +2625,7 @@ const char kHomepageSettingsUIConversionDescription[] =
     "homepage URL through a radio button group, and add promo box on NTP "
     "if homepage is not enabled.";
 
-const char kInstantStartName[] = "Enable instant start";
+const char kInstantStartName[] = "Instant start";
 const char kInstantStartDescription[] =
     "Show start surface before native library is loaded.";