Document and test that image_util.WritePngFile requires path end in 'png'.

tools/telemetry/telemetry/image_processing/image_util_unittest.py

Index: tools/telemetry/telemetry/image_processing/image_util_unittest.py
diff --git a/tools/telemetry/telemetry/image_processing/image_util_unittest.py b/tools/telemetry/telemetry/image_processing/image_util_unittest.py
index 0102e214b278aec268c7e1eca64a7f494c012226..540bf3e83edf2061d65f0fa838802a612ac2c78b 100644
--- a/tools/telemetry/telemetry/image_processing/image_util_unittest.py
+++ b/tools/telemetry/telemetry/image_processing/image_util_unittest.py
@@ -51,6 +51,12 @@ class ImageUtilTest(unittest.TestCase):
     new_file = image_util.FromPngFile(temp_file)
     self.assertTrue(image_util.AreEqual(orig, new_file, likely_equal=True))
 
+  def testWritePngWithoutPngSuffixThrows(self):
+    orig = image_util.FromPngFile(test_png_path)
+    temp_file = tempfile.NamedTemporaryFile().name
+    self.assertRaises(AssertionError, image_util.WritePngFile,
+                      orig, temp_file)
+
   def testWriteCroppedBmpToPngFile(self):
     pixels = [255,0,0, 255,255,0, 0,0,0,
               255,255,0, 0,255,0, 0,0,0]