Clarify the handling of unspecified url parameter to Network.getCookies

Bug: 1108556
Change-Id: I8d5100d313727aeee2adfa5a63bab599446114a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318314
Auto-Submit: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791388}

third_party/blink/public/devtools_protocol/browser_protocol.pdl

@@ -4860,7 +4860,9 @@ domain Network
   # detailed cookie information in the `cookies` field.
   command getCookies
     parameters
-      # The list of URLs for which applicable cookies will be fetched
+      # The list of URLs for which applicable cookies will be fetched.
+      # If not specified, it's assumed to be set to the list containing
+      # the URLs of the page and all of its subframes.
       optional array of string urls
     returns
       # Array of cookie objects.