Compare rounded sizes in CheckPreconditionsForAllocObject

Fixes a bug where the retrying the allocation for switching from
non instrumented -> instrumented would cause DCHECK failure because
it was comparing a rounded size to an unrounded size.

Bug: 37885600
Test: test-art-host
Change-Id: I2bffff6ae4e3c20b2e830a407a0688ba4cad51a1