Purge Skia objects from GL caches as needed.

Change-Id: I754c671cf790ad5ae8bf047ad328034217da4ecc
diff --git a/libs/hwui/TextureCache.h b/libs/hwui/TextureCache.h
index 847d69c..a63789a 100644
--- a/libs/hwui/TextureCache.h
+++ b/libs/hwui/TextureCache.h
@@ -87,6 +87,10 @@
     uint32_t mMaxSize;
     GLint mMaxTextureSize;
 
+    /**
+     * Used to access mCache and mSize. All methods are accessed from a single
+     * thread except for remove().
+     */
     mutable Mutex mLock;
 }; // class TextureCache