Start caching masks / stroke fills for textblobs

BUG=skia:

Review URL: https://codereview.chromium.org/1065293003
diff --git a/src/gpu/GrTextBlobCache.cpp b/src/gpu/GrTextBlobCache.cpp
index a5ef07c..fe0ccf6 100644
--- a/src/gpu/GrTextBlobCache.cpp
+++ b/src/gpu/GrTextBlobCache.cpp
@@ -10,7 +10,7 @@
 static const int kVerticesPerGlyph = 4;
 
 GrTextBlobCache::~GrTextBlobCache() {
-    SkTDynamicHash<BitmapTextBlob, uint32_t>::Iter iter(&fCache);
+    SkTDynamicHash<BitmapTextBlob, BitmapTextBlob::Key>::Iter iter(&fCache);
     while (!iter.done()) {
         (&(*iter))->unref();
         ++iter;