Search the entire subtree when looking for the end of an inline continuation chain.

Inlines may be nested, so we may not find the last inline in the chain as a
direct child of the anonymous blocks. We need to search the entire subtree.
Don't do this with anonymous blocks that wrap block children (the block-level
DOM children of the inline-level objects), though. We're not going to find
anything interesting there.

This fix is speculative; the original bug report didn't come with a test case.

BUG=662767

Review-Url: https://codereview.chromium.org/2738503004
Cr-Commit-Position: refs/heads/master@{#455420}