Unify the different chunk classes.

Instead of a series of alternating TextChunks and SplitChunks, each
chunk is now a piece of text following by split information. Cleans up
a bunch of stuff.

I was hoping it helped perf, but the effect ended up negligible. Still,
I think the resulting code is much cleaner and easier to reason about.

R=pquitslund@google.com

Review URL: https://chromiumcodereview.appspot.com//809623004