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See bug for detailed problem.
This change adds a new signal sent to the feed component, informing
that the feed is shown. This happens the first time feed content is
seen, even if a card is hardly shown.

Bug: 1117265
Change-Id: Ic0eb7fbb67a16b7f06554bc1ae48efa34662d106
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360600Reviewed-by: Ian Wells <iwells@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799253}