Add a description for --show-taps.

BUG=none

Change-Id: I37e27f6c46b5ee034ff3d508186a13494b7a1cac
Reviewed-on: https://chromium-review.googlesource.com/c/1461324
Commit-Queue: Avi Drissman <avi@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630530}

chrome/browser/flag-metadata.json

@@ -2872,6 +2872,8 @@
   {
     "name": "show-taps",
     "owners": [ "//ash/OWNERS" ],
+    // This is a debug flag, so that video bug reports can show input taps to
+    // aid in getting clear bug reports.
     "expiry_milestone": -1
   },
   {