Do not create stacking contexts for text fragments

Without this change, each text fragment in a block that establishes a
stacking context will establish its own stacking context. This is
unnecessary and increases the amount of work done during display list
construction.

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-input-015.htm.ini

@@ -1,3 +0,0 @@
-[transform-input-015.htm]
-  type: reftest
-  expected: FAIL