Avoid signed/unsigned warning in VC++ 2017 builds

VC++ 2017's STL doesn't suppress warnings as aggressively as prior
versions did. This causes warnings on code which mixes signed and
unsigned types. In this case a deque of unsigned integers was being
queried to see how many signed integers it contains. This could be
fixed by passing in unsigned 0, 1, and 2 to std::count but changing
the deque from unsigned to int is simpler.

R=adamk@chromium.org
BUG=chromium:683729

Review-Url: https://codereview.chromium.org/2834293002
Cr-Commit-Position: refs/heads/master@{#44814}

test/unittests/base/iterator-unittest.cc

@@ -42,7 +42,7 @@ TEST(IteratorTest, IteratorRangeArray) {
 
 
 TEST(IteratorTest, IteratorRangeDeque) {
-  typedef std::deque<unsigned> C;
+  typedef std::deque<int> C;
   C c;
   c.push_back(1);
   c.push_back(2);