Add types to implicit generics for JDK 7

Using <> is causing the tests not to build, see
http://ci.bazel.io/job/Bazel/387/.

--
MOS_MIGRATED_REVID=116254679

src/test/java/com/google/devtools/build/lib/util/StringIndexerTest.java

@@ -252,7 +252,8 @@ public void addStringResult() {
   public static class CanonicalStringIndexerTest extends StringIndexerTest{
     @Override
     protected StringIndexer newIndexer() {
-      return new CanonicalStringIndexer(new ConcurrentHashMap<>(), new ConcurrentHashMap<>());
+      return new CanonicalStringIndexer(new ConcurrentHashMap<String, Integer>(),
+          new ConcurrentHashMap<Integer, String>());
     }
 
     @Test