Check not to insert nullptr to HeapHashSet<WeakMember>

This patch ensures that nullptr is not inserted into CSSFontSelector and
FontCache::fontCacheClients(), in order to track de-referencing nullptr
on iterations.

If there were any such instances, we will crash early in a different
signature.

BUG=581698, 689949, 696231

Review-Url: https://codereview.chromium.org/2724753002
Cr-Commit-Position: refs/heads/master@{#453879}
2 files changed