DevTools: deprecate DOM.getFlattenedDocument command

DOMSnapshot.captureSnapshot should be used instead.

Change-Id: I32b9539e06b4f2d20bee6c34b7ba64a289c84ade
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335483Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795096}

third_party/blink/public/devtools_protocol/browser_protocol.pdl

@@ -2020,7 +2020,9 @@ domain DOM
       Node root
 
   # Returns the root DOM node (and optionally the subtree) to the caller.
-  command getFlattenedDocument
+  # Deprecated, as it is not designed to work well with the rest of the DOM agent.
+  # Use DOMSnapshot.captureSnapshot instead.
+  deprecated command getFlattenedDocument
     parameters
       # The maximum depth at which children should be retrieved, defaults to 1. Use -1 for the
       # entire subtree or provide an integer larger than 0.