[Font Load Events] Update FontFaceSet status when added/removed FontFace while loading

This patch tells FontFaceSet to adjust m_loadingCount when a FontFace is
added or removed while its status is "loading".

BUG=53213
TEST=fast/css/fontfaceset-add-remove-while-loading.html

Review URL: https://codereview.chromium.org/147953003

git-svn-id: svn://svn.chromium.org/blink/trunk@166287 bbb929c8-8fbe-4397-9dbb-9b2b20218538