Fix for GrAtlasTextContext memory leak

BUG=skia:

Review URL: https://codereview.chromium.org/1072193009
diff --git a/src/gpu/GrAtlasTextContext.h b/src/gpu/GrAtlasTextContext.h
index c0975b9..ebd94f9 100644
--- a/src/gpu/GrAtlasTextContext.h
+++ b/src/gpu/GrAtlasTextContext.h
@@ -252,6 +252,12 @@
             , fMinMaxScale(SK_ScalarMax)
             , fTextType(0) {}
 
+        ~BitmapTextBlob() override {
+            for (int i = 0; i < fRunCount; i++) {
+                fRuns[i].~Run();
+            }
+        }
+
         static const Key& GetKey(const BitmapTextBlob& blob) {
             return blob.fKey;
         }