Massage FontCache::getFontResourcePlatformData logic.

It's gotten quite hairy over time and I was having difficult time
reasoning about it, so FTFY.

BUG=315783
R=eae

Review URL: https://codereview.chromium.org/78053010

git-svn-id: svn://svn.chromium.org/blink/trunk@162659 bbb929c8-8fbe-4397-9dbb-9b2b20218538