Bug 983650. Temporarily mark b2g webgl reftests tests as random

These tests fail on the current test machines but pass on the new ones. Mark them as random for now until we finish the transition to the new machines.

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extra : rebase_source : 7a4b3d85ed7e7c6cf2081b623e776d9034385c90
Bug 977861. Avoid putting the scroll thumb at subpixel positions. r=mstange

The solution for fixing this properly is not clear and could be a lot of work.
Matt Woodrow agreed that this is probably the right thing to do for now.

This is b2g only because it was causing reftest failures on OS X that I do not
understand and is pretty important for performance.

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extra : rebase_source : 52517bae11ac4db46471143ff5a30a97c1860e33