Make sure blink scheduler doesn't wait for the PingLoader

The PingLoader calls context().willStartLoadingResource but it
doesn't call context().dispatchDidFinishLoading which throws off
the scheduler's accounting.  This patch fixes that.

BUG=546953

Review-Url: https://codereview.chromium.org/2215313003
Cr-Commit-Position: refs/heads/master@{#410173}
1 file changed