Oilpan: Prepare to move EventHandler to Oilpan heap.

BUG=

Review URL: https://codereview.chromium.org/317073007

git-svn-id: svn://svn.chromium.org/blink/trunk@175537 bbb929c8-8fbe-4397-9dbb-9b2b20218538
ServiceWorker: Add layout tests for fetch event response body

Multi-sided patch to implement blob-type fetch event 
response bodies:
1: blink-side, https://codereview.chromium.org/307063002/
2: chromium-side, https://codereview.chromium.org/315023003/
3: blink-side, THIS PATCH

BUG=379074

Review URL: https://codereview.chromium.org/304233017

git-svn-id: svn://svn.chromium.org/blink/trunk@175538 bbb929c8-8fbe-4397-9dbb-9b2b20218538