Export SkTextBlob::Iter

Inner classes aren't automatically exported if their outer class is.
This causes linking against SkTextBlob::Iter to fail on the MSVC
toolchain.

Example: https://github.com/google/skia/blob/master/include/core/SkBitmap.h
Failure: https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/406115
Change-Id: I1177afe5bde0d61b7b4604194f78664498683316
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/246579
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Calder Kitagawa <ckitagawa@google.com>
Auto-Submit: Calder Kitagawa <ckitagawa@google.com>
1 file changed