Test scaling for small images

We don't want to test small images on Gold because they are
not interested to look at.  Instead, I wrote a unit test to
verify that scaling small images does not cause crashes.

BUG=skia:

Review URL: https://codereview.chromium.org/1287863004
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 9826f97..f766b49 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -120,6 +120,12 @@
     if (size == decodeInfo.dimensions() && 1.0f != fScale) {
         return Error::Nonfatal("Test without scaling is uninteresting.");
     }
+
+    // Visually inspecting very small output images is not necessary.  We will
+    // cover these cases in unit testing.
+    if ((size.width() <= 10 || size.height() <= 10) && 1.0f != fScale) {
+        return Error::Nonfatal("Scaling very small images is uninteresting.");
+    }
     decodeInfo = decodeInfo.makeWH(size.width(), size.height());
 
     // Construct a color table for the decode if necessary