Add a comment to WTF::ParseDateFromNullTerminatedCharacters()

This CL has no behavior changes.

TBR=yhirano@chromium.org, yutak@chromium.org

Change-Id: I7d1a16b5760a20a3fd7e685aa248a28978fac491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564273Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649834}

third_party/blink/renderer/platform/wtf/date_math.h

@@ -53,6 +53,11 @@ namespace WTF {
 
 // Not really math related, but this is currently the only shared place to put
 // these.
+//
+// TODO(tkent): Only blink::ParseDate() uses this function to parse HTTP
+// header values. net::HTTPResponseHeaders::GetTimeValuedHeader() uses
+// base::Time::FromUTCString() for the same purpose.  We should consider
+// switching to base::Time::FromUTCString() for consistency.
 WTF_EXPORT double ParseDateFromNullTerminatedCharacters(
     const char* date_string);