Make sure the allocator can be unset when it is destroyed. This is
necessary when running tests, when each test recreates the allocator
instance. In case of views_aura_mus_unittests, this causes a number of
crashing failures, causing a lot of tests to be re-run separately.
Without this fix, the test step takes ~40 seconds to complete. With this
fix, it takes ~7 seconds.

BUG=none

Review-Url: https://codereview.chromium.org/2596903002
Cr-Commit-Position: refs/heads/master@{#440270}