Should check whether document.settings() is available or not when updating GenericFontFamilySettings.

Looking at crash reports - WebCore::StyleEngine::updateGenericFontFamilySettings, document().settings() is probably null. In the case, we should not update GenericFontSettings.

BUG=376525
TEST=It is very hard to reproduce crashes reported in crbug.com/376525. Will check # of crash reports to see whether the bug is really fixed or not.

Review URL: https://codereview.chromium.org/304033005

git-svn-id: svn://svn.chromium.org/blink/trunk@175783 bbb929c8-8fbe-4397-9dbb-9b2b20218538