Restructured alerts-history endpoint

Alerts history will return a list of IDs, which can then be used to get actual alerts from history. This allows to avoid exceeding memory and response size limits when processing the query.

BUG=416182
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/664563002

git-svn-id: svn://svn.chromium.org/blink/trunk@183845 bbb929c8-8fbe-4397-9dbb-9b2b20218538