Add a reference to chrome.loadtimes deprecation bug.

Remove the "TEMP" comment for WebLocalFrame::UsageCountChromeLoadTimes()
from 4 years ago.

Bug: 621512
Change-Id: If823b6685d73e3065e931a12e231380e6cd0833d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404523Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809439}

third_party/blink/public/web/web_local_frame.h

@@ -596,8 +596,8 @@ class WebLocalFrame : public WebFrame {
 
   // Events --------------------------------------------------------------
 
-  // TEMP: Usage count for chrome.loadtimes deprecation.
-  // This will be removed following the deprecation.
+  // Usage count for chrome.loadtimes deprecation.
+  // This will be removed following the deprecation. See: crbug.com/621512
   virtual void UsageCountChromeLoadTimes(const WebString& metric) = 0;
 
   // Whether we've dispatched "pagehide" on the current document in this frame