Speculative fix for stack overflow in CollectEvacuationCandidates

I don't have a repro, but it seems that when we are close to the stack limit,
CollectEvacuationCandidates' local variables can blow the limit. To avoid
this, instead of always allocating a 2000-pointer-sizes array on the stack,
use a std::vector (which stores its elements on the heap), and while we're
at it, only allocate as many elements as we actually need.

Review URL: https://codereview.chromium.org/1073123002

Cr-Commit-Position: refs/heads/master@{#27742}