Export SkTextBlob::Iter
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 <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>
Reviewed-by: Calder Kitagawa <a24666410af2d9c64adf9f6c273b074bce0adf22@google.com>
Auto-Submit: Calder Kitagawa <a24666410af2d9c64adf9f6c273b074bce0adf22@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,google/skia,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc