Add approxBytesUsed to hashes.

BUG=skia:

Review URL: https://codereview.chromium.org/1280653003
diff --git a/tests/HashTest.cpp b/tests/HashTest.cpp
index 8489861..f53faaf 100644
--- a/tests/HashTest.cpp
+++ b/tests/HashTest.cpp
@@ -23,6 +23,8 @@
     map.set(3, 4.0);
     REPORTER_ASSERT(r, map.count() == 1);
 
+    REPORTER_ASSERT(r, map.approxBytesUsed() > 0);
+
     double* found = map.find(3);
     REPORTER_ASSERT(r, found);
     REPORTER_ASSERT(r, *found == 4.0);