Add 'didDispatchPingLoader' callback to FrameLoaderClient.

Our current <a ping> and CSP violation report layout tests dispatch a
PingLoader to a script that writes to a file, and then navigate to
another script that reads that file and dumps the payload. This works
for most cases, but doesn't allow us to effectively assert that the
number of pings matches the expectations.

Adding a 'didDispatchPingLoader' callback enables visible output in
layout tests when a PingLoader is dispatched to a URL, which makes it
possible to verify that we're sending one and only one ping to a URL.

This will be particularly useful for a future CL which intends to
deduplicate CSP violation reports.

Review URL: https://chromiumcodereview.appspot.com/22159002

git-svn-id: svn://svn.chromium.org/blink/trunk@155608 bbb929c8-8fbe-4397-9dbb-9b2b20218538