Oilpan: Fix leak from FontLoader.

Prior to this patch, FontLoader referenced ResourceFetcher as a Member, but this is wrong, as FontLoader lifetime and ResourceFetcher lifetime is not correlated.

Font->CSSFontSelector->FontLoader can outlive Document when cached by FontCache.

ResourceFetcher may belong to Document OR DocumentLoader.
Their lifetime is independent from (cached) Fonts lifetime.

This caused leaks in svg/batik/paints/*.svg LayoutTests.

TEST=svg/batik/paints/gradientLimit.svg w/ --enable-leak-detection
BUG=None

Review URL: https://codereview.chromium.org/339663006

git-svn-id: svn://svn.chromium.org/blink/trunk@176814 bbb929c8-8fbe-4397-9dbb-9b2b20218538