Move FP texture test buffers to heap.

This is a speculative fix for the crashing N5 bots.  It looks like the bots are
always failing in or around FloatingPointTextureTest.

It looks like sometimes they're hitting a SIGBUS, which I suspect is stack
overflow.  FloatingPointTextureTest allocates ~320K on the stack, which may
be too much.  This CL moves those buffers to the heap.  For consistency I did
the same with the half-float tests, though they're only using ~1/8th the stack.

It looks like sometimes the bots are failing to malloc.  I don't understand that,
and this CL doesn't address that directly.  But it's possible this is still a stack
overflow, just trashing RAM and causing arbitrary mayhem instead of a SIGBUS.

I have no idea why this is a problem only on the N5.  I have been unable to
reproduce this locally, neither with a K N5 nor an L N5, but the bots are pretty
reliable.

NOTREECHECKS=true
BUG=skia:

Review URL: https://codereview.chromium.org/871623002

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