Add test to constellation to avoid writing reftest image if there are pending frames.

Also change when pipelines become active.

This makes the constellation activate a pipeline as the current frame
when it is ready to do initial reflow, rather than when it is ready
to paint.

This fixes a number of intermittent failures that could previously occur
if an iframe was not visible - which would mean it was never moved from
a pending frame in the constellation to an active frame.

(It happens that webrender exposes these intermittents as permanent failures).

tests/wpt/metadata-css/css21_dev/html4/root-box-003.htm.ini

@@ -1,3 +1,3 @@
 [root-box-003.htm]
   type: reftest
-  expected: TIMEOUT
+  expected: FAIL