To reproduce:
1. Open attached project
2. Open "BugRepo" scene
3. Enter play mode
4. Open Profiler and enable deep profile
5. Notice allocations under JsonTest.Update()

Using JsonUtility.FromJsonOverwrite to deserialize objects with array/list fields, unexpected/inconsistent GC alloc happens.

Cases:
1) deserializing into array with same number of elements
2) deserializing into array with different number of elements
3) deserializing into list with same number of elements
4) deserializing into list with different number of elements

Actual results:
1) no alloc
2) new array allocated, then each element in new array allocated
3) GC.Alloc call (list reference on my object doesn't change)
4) list ref instance is the same, but more allocations than case (2)

Expected results:
1) no alloc (ok)
2) array reallocated, added elements allocated if needed (old elements replaced)
3) no alloc
4) no alloc if list Capacity is enough to contain new items

Reproducible: 2018.3.0a1, 2018.2.0b1, 2018.1.0f2, 2017.4.2f1