Do not schedule font loading if already started

r162179 (reverted in r162249) caused timeouts on some layout tests where
onload handler was not invoked. That happens when
FontLoader::beginLoadTimerFired does not start any font loading
because all fonts in m_fontsToBeginLoading are already loading.
ResourceFetcher::decrementRequestCount makes the request count zero, but
it does not trigger the load event.

To resolve this, this patch makes FontLoader::addFontToBeginLoading
skip fontResource if it's already loading or loaded.

BUG=246492,320732
TEST=fast/frames/seamless/seamless-nested-crash.html,http/tests/misc/acid3.html

Review URL: https://codereview.chromium.org/75983002

git-svn-id: svn://svn.chromium.org/blink/trunk@162329 bbb929c8-8fbe-4397-9dbb-9b2b20218538