Update app_units to 0.3.1

The height test is removed in mime_sniffing_font_context because of rounding
changes in app_units. The rest of that test is still useful though, given it
will timeout if the font fails to be sniffed.

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-017.htm.ini

@@ -1,3 +1,4 @@
 [transform-input-017.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": FAIL

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-018.htm.ini

@@ -1,3 +1,4 @@
 [transform-input-018.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "linux": FAIL