Remove run count field from SkTextBlob.

Remove run count field from SkTextBlob.

We can flag the last run record instead.  Run iteration is always
sequential, so no penalty.

As a side effect, we can no longer allow instantiation of zero-run text
blobs - but that seems like a good idea anyway.

Change-Id: I7ca80c4780623d5a188f92dfe6d6fe152f20f666
Reviewed-on: https://skia-review.googlesource.com/9149
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>