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Otherwise chrome:// are not recognized as the search result.

This CL also modifies the logic of URLSearchProvider's result
aggregation, because the SearchProvider doesn't emit result change
for URL-ish results.

BUG=416083
R=oshima@chromium.org
TBR=mpearson@chromium.org, agl@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/584263002

Cr-Commit-Position: refs/heads/master@{#295803}