Decreased num alerts in history per page to 5 to avoid exceeding memory and response limit

R=machenbach@chromium.org,ojan@chromium.org
BUG=416182

Review URL: https://codereview.chromium.org/657533002

git-svn-id: svn://svn.chromium.org/blink/trunk@183738 bbb929c8-8fbe-4397-9dbb-9b2b20218538