Remove PseudoDisplayItemClass

[mrobinson]

Now that content box queries are made against the flow tree, we can
remove PseudoDisplayItems from the display list.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3252

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[jdm]

Interestingly, both mac and linux reftests failed with background_repeat_x_a.html == background_repeat_x_b.html.

[mrobinson]

So after a lot of false starts I was finally able to track the test failure down to a bug in how we specify the clipping region during display list rasterization. Previously clips were applied before the transformation matrix, which is problem for positioned children and non-origin tiles. Removing PseudoDisplayListItem merely uncovered this bug.

[pcwalton]

r+

[bors-servo]

saw approval from pcwalton
at mrobinson@2e8f1c0

[bors-servo]

merging mrobinson/servo/pseudo-rm-rf = 2e8f1c0 into auto

[bors-servo]

mrobinson/servo/pseudo-rm-rf = 2e8f1c0 merged ok, testing candidate = d988d01

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/242
success: http://build.servo.org/builders/linux2/builds/242
success: http://build.servo.org/builders/android/builds/309
success: http://build.servo.org/builders/mac1/builds/459
success: http://build.servo.org/builders/mac2/builds/459

[bors-servo]

fast-forwarding master to auto = d988d01