Explain what 'tabs' and 'cbd' mean in the TabsInCBD flag

Change-Id: Ifccf02402e0513c9e2c96dad335d05ab23b705ef
Reviewed-on: https://chromium-review.googlesource.com/574919Reviewed-by: Anita Woodruff <awdf@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487469}

chrome/common/chrome_features.cc

@@ -96,6 +96,7 @@ const base::Feature kTabStripKeyboardFocus{"TabStripKeyboardFocus",
                                            base::FEATURE_DISABLED_BY_DEFAULT};
 #endif  // defined(OS_MACOSX)
 
+// Enables Basic/Advanced tabs in ClearBrowsingData.
 const base::Feature kTabsInCbd {
   "TabsInCBD",
 #if defined(OS_ANDROID)