[oilpan]: Move ResourceFetcher to the oilpan heap to get rid of persistents in Document.

Used ResourceLoader::RequestCountTracker to get around the inc/dec of the request count in CSSFontSelector.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/261263004

git-svn-id: svn://svn.chromium.org/blink/trunk@173397 bbb929c8-8fbe-4397-9dbb-9b2b20218538