FontTemplateData gets passed over IPC during the communication between
FontContext and FontCacheThread. Serializing and deserializing these
bytes is expensive, so this change ensures that we only do that when the
bytes can't be read from disk. A similar strategy is already used on
macos and windows.

The performance problem was particularly noticeable after implenting
font fallback, where the content process would potentially work through
a list of fonts, trying to find one which contains a certain glyph. That
could result in lots of font bytes going over IPC.