CSP: Deduplicate violation reports before sending.

Violation reports should be sent once and only once per page load. If a
single line of code generates the same report over and over again, we
should attempt to avoid spamming the report server with not-particularly
valuable duplicates.

For example, if a report-only policy blocks 'eval()', then the following
loop would make a sysadmin somewhere quite unhappy:

    for (i=0;i<Number.MAX_VALUE;i++)
      eval(...);

This patch adds a HashSet<unsigned> to ContentSecurityPolicy, and stores
the hash of the stringified violation report. We check that set just
before handing things off to PingLoader for delivery. If there's a
match, we've already sent the report, and can safely discard it. If not
we send the report, then add it to the list.

Discussed on public-webappsec@w3.org: http://lists.w3.org/Archives/Public/public-webappsec/2013Aug/0000.html

BUG=267453

Review URL: https://chromiumcodereview.appspot.com/21789002

git-svn-id: svn://svn.chromium.org/blink/trunk@155708 bbb929c8-8fbe-4397-9dbb-9b2b20218538