Add isReload attribute to FetchEvent [blink] (1/3)

http://www.w3.org/TR/service-workers/#is-reload-attribute

Multi-sided patch:
1/3: blink-side, THIS PATCH
2/3: chromium-side, https://codereview.chromium.org/341043002/
3/3: blink-side, change IDL,  Not Yet.

BUG=386507

Review URL: https://codereview.chromium.org/332903003

git-svn-id: svn://svn.chromium.org/blink/trunk@176583 bbb929c8-8fbe-4397-9dbb-9b2b20218538