[ServiceWorker] Disable the cache revalidation when the page is controlled by the ServiceWorker.

The revalidation headers ("if-modified-since", "if-none-match") should not be exposed to the ServiceWorker.

BUG=429570
TEST=http/tests/serviceworker/resources/fetch-request-no-freshness-headers-iframe.html

Review URL: https://codereview.chromium.org/703813002

git-svn-id: svn://svn.chromium.org/blink/trunk@184864 bbb929c8-8fbe-4397-9dbb-9b2b20218538