add textblob cache freeall

BUG=skia:

Review URL: https://codereview.chromium.org/1088683004
diff --git a/src/gpu/GrTextBlobCache.h b/src/gpu/GrTextBlobCache.h
index a110a03..1776c69 100644
--- a/src/gpu/GrTextBlobCache.h
+++ b/src/gpu/GrTextBlobCache.h
@@ -115,6 +115,8 @@
         fBlobList.addToHead(blob);
     }
 
+    void freeAll();
+
 private:
     // TODO move to SkTextBlob
     void BlobGlyphCount(int* glyphCount, int* runCount, const SkTextBlob* blob) {