Force a reflow after adding a stylesheet

[mbrubeck]

This fixes a race (exposed in some reftests by #3563) where the document's
first paint happens before the stylesheet is added, and it never updates
after the stylesheet loads.

r? @jdm

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2828

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[jdm]

r+

[bors-servo]

saw approval from jdm
at mbrubeck@5011fd3

[bors-servo]

merging mbrubeck/servo/relayout = 5011fd3 into auto

[bors-servo]

mbrubeck/servo/relayout = 5011fd3 merged ok, testing candidate = 9024eef

[bors-servo]

some tests failed:
failure: http://build.servo.org/builders/linux/builds/195
failure: http://build.servo.org/builders/mac/builds/197

[mbrubeck]

The test that failed on Linux was the exact one this was intended to fix. So I guess it doesn't have the desired effect. :/ Closing for now, and we should probably disable the test until it can be fixed for real.