Cleanup and improve supportsPositionFixed

Support position fixed on Trident (newer IE versions) and add some tests.

Remove irrelevant FIXMES - test show Webkit examples are accounted for.
Note Blink is Chrome 28+ which is Webkit based. Not sure why these
tests would thus fail with Blink. FIXME should at least include a counter
example.

Bug: 65118
Change-Id: I09959b4fe0151009cbebbcc40e00a9d1df85187b
Merge "Cleanup and improve supportsPositionFixed"