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This also changes the way DomDistillerResult is read from base::Values by
updating to the new version of DomDistillerResult::ReadFromValue which
takes in a pointer to DomDistillerResult.

This CL also makes it possible to continue using the
update_domdistiller_js.sh script without modification by moving the
BUILD.gn file for protos to outside of the generated package folder.

Picked up changes:
432b2dc Added support for reading protos into a pointer.
32f096c don't prune a subtree when its only child is a text node.
90dc83c bump up nested-table heuristic
6f59530 Make comment filter a little more restrictive.

BUG=368941,408292,409274

Review URL: https://codereview.chromium.org/514423002

Cr-Commit-Position: refs/heads/master@{#293275}