Use History API in Sheriff-O-Matic.

It allows the sheriff to use back and forward buttons.

BUG=None

Review URL: https://codereview.chromium.org/478453004

git-svn-id: svn://svn.chromium.org/blink/trunk@181089 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Simplify XMLHttpRequestProgressEventThrottle event firing.

This CL doesn't change the behavior except that previous implementation
dispatches a "progress" Progress event when an XHR gets "DONE" with some error
in very rare case, but with this CL that event is cleared.

BUG=406397

Review URL: https://codereview.chromium.org/505953003

git-svn-id: svn://svn.chromium.org/blink/trunk@181090 bbb929c8-8fbe-4397-9dbb-9b2b20218538