Mark vertical-lr-blocks.html as flaky to ease the pain while it's being ...

[jdm]

...investigated. #3926

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3177

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[Ms2ger]

r+ p=1

[bors-servo]

saw approval from Ms2ger
at jdm@a589ed7

[bors-servo]

merging jdm/servo/flakyvertical = a589ed7 into auto

[bors-servo]

jdm/servo/flakyvertical = a589ed7 merged ok, testing candidate = 80baba0

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux/builds/643
success: http://build.servo.org/builders/mac1/builds/185
success: http://build.servo.org/builders/mac2/builds/185

[bors-servo]

fast-forwarding master to auto = 80baba0

[pcwalton]

For what it's worth I think the issue may be something to do with style sharing related to the sibling selectors it uses.