Store iterator bitset by-value instead of by-reference.

The by-reference method was giving strange problems when using a non-
LValue as argument to IterateBitSet.

BUG=angleproject:1040

Change-Id: Iec10f7256a5b19b239804e0beba4ae675f53f876
Reviewed-on: https://chromium-review.googlesource.com/293840
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/common/BitSetIterator.h b/src/common/BitSetIterator.h
index d0cc7c0..3248ce4 100644
--- a/src/common/BitSetIterator.h
+++ b/src/common/BitSetIterator.h
@@ -52,7 +52,7 @@
     Iterator end() const { return Iterator(std::bitset<N>(0)); }
 
   private:
-    const std::bitset<N> &mBits;
+    const std::bitset<N> mBits;
 };
 
 template <size_t N>