fix for atlas is abandoned text corruption

BUG=skia:

Review URL: https://codereview.chromium.org/1156693004
diff --git a/src/gpu/GrBatchFontCache.cpp b/src/gpu/GrBatchFontCache.cpp
index a1f87cb..2d6213c 100644
--- a/src/gpu/GrBatchFontCache.cpp
+++ b/src/gpu/GrBatchFontCache.cpp
@@ -75,6 +75,7 @@
 GrBatchFontCache::~GrBatchFontCache() {
     SkTDynamicHash<GrBatchTextStrike, GrFontDescKey>::Iter iter(&fCache);
     while (!iter.done()) {
+        (*iter).fIsAbandoned = true;
         (*iter).unref();
         ++iter;
     }
@@ -86,6 +87,7 @@
 void GrBatchFontCache::freeAll() {
     SkTDynamicHash<GrBatchTextStrike, GrFontDescKey>::Iter iter(&fCache);
     while (!iter.done()) {
+        (*iter).fIsAbandoned = true;
         (*iter).unref();
         ++iter;
     }