PM: Plumb page favicons to chrome://discards graph.

Change-Id: I22847d87db876b27bc424fc53afdccf1d85b4903
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611112


Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660543}