Make SkTextBlob::RunIterator public.

Motivation: This will be easier than adding a friend every time I want
to create a one-off SkCanvas subclass or SkRemote::Encoder subclass.

See also: SkPath::Iter.

Review URL: https://codereview.chromium.org/1411723005
diff --git a/tests/TextBlobTest.cpp b/tests/TextBlobTest.cpp
index 027fd2a..aa4b1ea 100644
--- a/tests/TextBlobTest.cpp
+++ b/tests/TextBlobTest.cpp
@@ -7,7 +7,7 @@
 
 #include "SkPaint.h"
 #include "SkPoint.h"
-#include "SkTextBlob.h"
+#include "SkTextBlobRunIterator.h"
 
 #include "Test.h"
 
@@ -196,7 +196,7 @@
 
         SkAutoTUnref<const SkTextBlob> blob(builder.build());
 
-        SkTextBlob::RunIterator it(blob);
+        SkTextBlobRunIterator it(blob);
         for (unsigned i = 0; i < outCount; ++i) {
             REPORTER_ASSERT(reporter, !it.done());
             REPORTER_ASSERT(reporter, out[i].pos == it.positioning());